It started with a bleary-eyed Google search: "Sell breast milk." Desiree Espinoza had a 2-month-old baby girl but was pumping out enough milk to feed triplets. Ziplock baggies full of the stuff were crammed in her freezer, and unpaid bills crowded her kitchen table. She wasn't sure there was a market for her overflow or whether selling it was even legal. A few clicks later, she found herself on a website called Only the Breast.

The site looks a lot like craigslist, except instead of selling used cars and like-new Ikea furniture, Only the Breast deals in human breast milk. There are hundreds of posts from new mothers eager to turn their surplus into profits. Many kick off with a chirpy headline ("Chubby baby milk machine!"), then follow with a snapshot of their own robust infant and lush descriptions ("rich, creamy breast milk!" "fresh and fatty!"), making a primal source of nutrition sound like a New York cheesecake. The posts are additionally categorized to appeal to a variety of milk seekers, based on a baby's age (from 0 to 12 months), say, or special dietary restrictions (dairy- and gluten-free). There's also a sort of "anything goes" section for women willing to sell to men. Some ship coolers of frozen milk packed in dry ice. Others deal locally, meeting in cafés to exchange cash for commodity. The asking price on Only the Breast runs $1 to $2.50 an ounce. (A 6-month-old baby consumes about 30 ounces a day.)

Liquid Gold: The Booming Market for Human Breast Milk by Judy Dutton (52.5 MB .mp3)

Subscribe: Wired Features PodcastIntrigued, Espinoza tapped out her sales pitch: "Mostly organic raised breast milk. I have over 500 oz saved and I need to get rid of it. During the week I only eat organic." A few days later, she was in business, selling the milk at $2 an ounce to a couple of customers in the Phoenix area where she lives, including a mother with a newborn and a man who claimed breast milk helped his immune disorder. "There's no way I could get a job with an infant, so this helps pay for diapers and clothes," she says. In three months, the 19-year-old college student earned enough to buy a new laptop and the dress she wore to her wedding to the baby's 22-year-old father, a recent college grad. She plans to continue selling for a year, and if she can pump a steady 30 ounces a day, she could take in about $20,000.

Only the Breast represents just one facet of the emerging market in human milk. In an era when the benefits of breast milk are better understood and more scientifically certain than ever, demand for it has created a niche industry. Besides sites like Only the Breast, that demand is being met by a handful of all-volunteer women's groups that help organize free milk donations via Facebook and their own websites. Two prominent ones, Human Milk 4 Human Babies and Eats on Feets (a play on Meals on Wheels), connect thousands of women, facilitating the donation of raw or home-pasteurized milk to new moms in need.

There's also a well-established brick-and-mortar network of so-called milk banks. These nonprofit operations collect milk from donors and process and pasteurize it to meet certain quality and safety standards. The milk is sold mostly to hospitals and parents of sick or premature infants at around $4 an ounce. A newer player is Prolacta Bioscience, a for-profit enterprise that operates somewhat like a pharmaceutical company, with a large-scale plant in Southern California. Prolacta produces its own enhanced breast-milk product, a syrupy fortifier specifically for hospitalized newborns, at a cost of $135 per baby, per day. With 58 hospital contracts and an ambitious distribution strategy for the next year, Prolacta envisions a multimillion-dollar opportunity for its products.

Most body fluids, tissues, and organs—semen, blood, livers, kidneys—are highly regulated by government authorities. But not breast milk. It's considered a food, so it's legal to swap, buy, or sell it nearly everywhere in the US. This accounts, in part, for the widely varying quality and safety standards in the online market for milk. For their part, Prolacta and nonprofit milk banks have rigorous screening processes for potential donors, including tests for drugs, hepatitis, and HIV. But Only the Breast and the volunteer sites, which see themselves more as communities than commodity markets, don't screen donors or assume responsibility for the milk they help disseminate.

Whatever the source of the milk or its channel of distribution, the trend is clear: Human milk is being bought, sold, donated—and gratefully received—on an unprecedented scale. And as demand grows, the competition for every ounce is getting more fierce.

The overall benefit of feeding babies breast milk instead of formula has been well established. In 2007, the US Department of Health and Human Services issued a report showing that babies who are formula-fed instead of breast-fed are at an increased risk for asthma, acute ear infections, diarrhea, and SIDS. 1 The advantages of feeding breast milk to babies are touted by some to be lifelong, potentially lowering the odds of obesity and boosting IQ by as much as 5 points.

Researchers have only recently begun to identify the mechanisms underlying breast milk's powerful effects. Look at it through a microscope and you can see that breast milk is abuzz with white blood cells, pearly fat globules, and fuzzy balls of protein. At higher magnification, you can make out the millions of Y-shaped molecules that are an infant's primary defense against infection: antibodies. Produced by the mother's immune system in response to the pathogens in her environment, these antibodies are passed along to the baby to fight off illness. Mothers' milk has other protective properties and potential uses as well. Sugars called oligosaccharides, long thought to have no function, since infants can't digest them, are now known to adhere to a baby's intestinal lining, allowing good bacteria in while repelling harmful bugs, like a discerning bouncer at a hip club. Fatty acids called DHA and AA serve as brain food, stimulating neurological development. One fatty acid-protein hybrid nicknamed Hamlet (Human Alpha-lactalbumin Made Lethal to Tumor Cells) has been found to kill 40 different types of cancer cell lines in the lab and is being researched as a treatment for patients. Breast milk also contains a host of stem cells. While scientists don't know yet what they're doing there, researchers suspect they may have the ability to differentiate into disease-fighting agents and could one day be harvested to treat an array of ailments, thus sidestepping the ethical concerns of harvesting stem cells from human embryos.

The multitasking magic of breast milk has proven difficult to replicate. J. Bruce German, a professor of food chemistry at UC Davis, has studied breast milk for two decades with the aim of mimicking certain effects. "The features that make human milk so unusual are that it's personalized and it's active," German says. "So almost by definition there's nothing on the horizon that would satisfy those criteria."

While the scientific understanding of human milk is still evolving, the American Academy of Pediatrics recommendation is straightforward: Mothers should feed their babies nothing but breast milk for the first six months and then continue nursing for at least another six. But the physical demands of breast-feeding and the time required to keep up with it can be daunting for new moms. (Breast-feeding can take as much as four hours a day; a pumping session takes, on average, 15 minutes and yields 6 ounces.) According to the Centers for Disease Control and Prevention and a report called the Ross Mothers Survey, breast-feeding at birth is up from approximately 25 percent in the early 1970s to 75 percent in 2007. But less than half of all mothers make it to the recommended six-month mark, and just a fifth stick with at least some breast-feeding for an entire year. Still, those moms who don't breast-feed want what's best for their babies, and many are willing to do whatever it takes to get it—including buying it from strangers online.

The online purveyors of mothers' milk see themselves as brokering an entirely wholesome trade. "We love that our site has already helped so many moms and babies, and we hope we can continue to connect them together for years to come," says Chelly Snow, cofounder of Only the Breast. She came up with the idea after giving birth in January 2009 and reading articles online about the benefits of breast-feeding. She kept spotting the same sort of posts in the comments section of these articles: "I need breast milk. Where do I get it?" and "I have too much. What do I do with it?" Both craigslist and eBay had long ago banned the sale of bodily fluids. Snow saw buyers and sellers who weren't connecting, and the idea for a website that filled that gap—and her bank account—was born.

Since she and her husband, Glenn, launched the site a year and a half ago, Only the Breast has attracted roughly 3,000 members (posting is free but both sellers and buyers must register to make contact). The couple, who run the business out of their Walnut Creek, California, home, plan to bring in revenue by selling ads. For now, though, they post their own endorsements for such things as prenatal vitamins and baby sign-language classes. They have yet to turn a profit, but their enterprise has already expanded, with recently launched sister sites in the UK and the European Union.

Meanwhile, the donation-based milk-sharing sites—particularly Eats on Feets, which attracts a lot of Whole Foods-shopping earth mamas—see what they do as the continuation of an age-old practice. Women have breast-fed one another's babies for millennia, they point out, and Internet-enabled milk swapping is just a 21st-century update. The FDA doesn't see it in such benign terms. In November 2010, the agency issued a stern press release warning about the risks of feeding someone else's bodily fluids to your baby: "When human milk is obtained directly from individuals or through the Internet, the donor is unlikely to have been adequately screened for infectious disease or contamination risk. In addition, it is not likely that the human milk has been collected, processed, tested, or stored in a way that reduces possible safety risks to the baby."

Despite the FDA's caution, there have been no reported cases of infection from breast milk acquired online. And those who use these sites say it's not really the government's place to step in. Also, by skirting the steep cost of screening and processing, which FDA-endorsed banks must bear, women who sell their milk on these sites can charge a relatively low price. As a result, they say, they can have a far greater impact on children's health than they would if they donated to a milk bank.

The online outlets also funnel donated milk to women like Kristen Conklin-Leveille, a 24-year-old from Ballston Spa, New York, who developed a staph infection soon after giving birth, rendering her unable to breast-feed. She tried four different kinds of formula, but her 2-month-old son had trouble digesting them. She tried goat's milk, but he refused it. She looked into buying from a nonprofit milk bank, but at $5 an ounce, it could have cost $150 a day, or more than $50,000 a year. Desperate, she posted a plea for milk donations on the Eats on Feets message board and breast milk started trickling in. "A lot of women told me, 'I was going to give this to a milk bank,'" Conklin-Leveille says, "'but I'd rather give it to you.'"

Women may prefer to donate their milk to other moms than to give it to a milk bank, which will sell it for $4 an ounce. And mothers seeking milk are happy to take those donations, especially since health insurance doesn't cover breast milk. "Until we have national regulations where insurance companies pay for donor milk across the board, we're going to continue to have more and more of this informal sharing," says Lois Arnold, program coordinator for the National Commission on Donor Milk Banking, a program of the American Breastfeeding Institute. No wonder milk banks are struggling to maintain their supply.

"By anyone's estimation, there's a milk shortage," says Kim Updegrove, executive director of the Mother's Milk Bank at Austin. It's located inside a quiet office park between two obstetrical practices. (The thinking is that expectant moms will eventually become donors.) But Updegrove and her colleagues are feeling pinched. This bank used to have 14,000 ounces in frozen storage for distribution to premature babies in hospitals and to parents with sick infants at home. It currently has just one-tenth of that amount.

Part of the problem is that the bank's self-imposed screening requirements tend to put off busy, exhausted new mothers. Potential donors must submit to a verbal and written questionnaire about their health history and present evidence of a postbirth blood test showing that they don't have an infectious disease like HIV. Since most of the milk from banks goes to preemies, it must also be free of over-the-counter medications and herbal supplements, including fenugreek (often taken to increase breast-milk production). The approval can take several days, giving a woman plenty of time to find more immediate online recipients. And if OK'd, a woman must make a minimum donation of 100 ounces, which further eliminates mothers who have more modest offerings. Half of the women who offer to donate do not complete the process—some because they don't pass the screening tests, others because they choose to share their milk more informally for less hassle.

Collecting the milk is just the beginning of a lengthy process that also involves analyzing, purifying, and standardizing the product. Once a donation has been tested for microbes and found to be pure, a small sample is analyzed by a machine called a MilkoScan FT 120. It looks like an espresso maker and uses an interferometer to scan the milk. The result is a readout showing the proportion of protein, fats, lactose, and calories, which can vary from 12 to 38 per ounce. To sell to hospitals, banks must provide milk with a specific, consistent balance of nutrients and calories. To achieve that, the banks use a technology called target pooling—blending donations with various nutrient profiles until the optimal caloric value (around 20 calories per ounce) is achieved. Finally, they pasteurize the milk with a special technique that heats it to about 144.5 degrees Fahrenheit, minimizing the destruction of antibodies while killing viruses and other pathogens.

Screening milk donors turns up a surprising number of infectious agents—pathogens that could be passed on to a baby. A 2010 Stanford University study examined data from 1,091 women who applied to donate milk to a bank in San Jose, California. It revealed that 3.3 percent were rejected after their blood samples tested positive for at least one of five serious infections: syphilis, HIV, hepatitis B, hepatitis C, and human T-cell lymphotropic virus. And if these pathogens are in a donor's blood, they can be present in the milk, too.

Critics warn that these same disease agents are likely prevalent in milk that's offered online. "Women are convinced that breast milk is somehow different from blood and that there aren't any risks in sharing it with another woman's baby," says Updegrove, director of the milk bank in Austin. "But it's an incredibly risky practice. Breast milk is a body fluid. Would you consider cutting open a vein and giving a direct transfusion?"

Given the appeal of their safety standards, banks aren't in danger of closing tomorrow. But they're not equipped to meet the current demand for human milk. And it's about to get worse: Not only is the rise of online selling and sharing chipping away at their supply, but Prolacta, the large-scale manufacturer of fortified milk products, has launched its own aggressive milk-soliciting campaign.

Inside a stainless steel tank the size of a Jacuzzi, milk spins in a high-speed centrifuge that screams like a jet engine. Meanwhile, two men in germ-free clean suits adjust various dials studding the tank's exterior. The operation—separating the constituent parts of 160 gallons of human milk—is an early step in what will soon transform the liquid into Prolact+4 H2MF, a dense syrup that packs the power of 10 ounces of regular milk into a single ounce. Higher-strength versions are also available, including Prolact +6, +8, and +10. On average, a 10-week supply of Prolacta Bioscience's product costs $10,500 per baby.

Prolacta is the first corporation to develop and sell breast milk for profit. Located in Monrovia, California, its 15,000-square-foot facility houses millions of dollars' worth of pristine ultrafiltration units, temperature probes, and an electrophoresis genetic analyzer. Founded in 1999, the company has 36 employees, $25 million in venture capital, and an expanding roster of hospital clients across the country. 2 Business is good, CEO Scott Elster says. He won't disclose exact figures but says sales grew by 200 percent in 2010; he expects a similar jump this year.

Prolacta started out with a business model that looked something like a for-profit milk bank, selling its donated supply to hospitals, where it was used mostly for infants in neonatal intensive care units. Many preemies were fed milk from their own mothers instead, boosted with a nutrient-rich infant fortifier. But since this fortifier was made from cow's milk, it often caused adverse effects, such as allergic reactions or digestive problems. The researchers at Prolacta wondered whether an infant fortifier from human milk would cause fewer problems. The company soon switched its focus to creating a human-derived milk fortifier, ran clinical trials, and by 2008 began marketing the product to hospitals.

The company stays fully stocked with donated milk, Elster says, thanks to a team of paid mommy recruiters who cover the US. Alice Toth and Keri Pommerenk, twin sisters (and now mothers), run Prolacta's Milkin' Mamas milk depot in Huntington Beach, California. The twins solicit donors by distributing flyers at hospitals and doling out $20 gift certificates to various baby-supply stores.

Prolacta does not compensate donors for fear that a financial incentive will attract women desperate enough to work the system. "We have to make it altruistic," Elster says. "Otherwise, there'll be a picture of a mom on the front page of The New York Times saying, 'I sold my milk for crack.'" (There's a history of such publicized scandals: When nonprofit milk banks first started, they paid donors. But that practice stopped in the early 20th century, after mothers began diluting their donations with cow's milk.)

With the right recruiting tactics, Prolacta can more than meet demand using volunteers, Elster says. "There's enough milk out there," he says. "Our main hold-up is educating the market"—that is, persuading more hospitals to buy Prolacta's superserum and persuading more mothers to donate.

But online sellers are now educated enough to see this as a raw deal. To women like Desiree Espinoza, who is making a tidy profit, it doesn't make sense to hand her business over to a milk bank or pharmaceutical company when her target market—mothers with hungry babies—can already get what they need from her for much cheaper. "I'm not embarrassed by what I do," Espinoza says. "This is my thing. I have an opportunity to help people, so why not?"

Judy Dutton (judy@judy-dutton.com) is the author of Science Fair Season: Twelve Kids, a Robot Named Scorch ... and What It Takes to Win (Hyperion).

Note 1. Correction appended [9:21 a.m. PDT/June 9, 2011]: The government report about formula-fed babies was published in 2007, not 2004. Prolacta was founded in 1999, not 2005.

Note 2. Correction appended [9:21 a.m. PDT/June 9, 2011]: Prolacta was founded in 1999, not 2005.