When Julie Peterson decided to have a baby on her own two years ago, she picked a tall, blond, blue-eyed Danish engineer as a sperm donor to match her own Scandinavian heritage. But when she went back to the sperm bank to use the same donor to have another child, she was stunned to discover that the federal government had made it impossible.

"I just cried," said Peterson, 43, who lives in North Carolina. "I was in complete shock. I hadn't thought about anything but having another baby with this donor. It was just so surprising and bewildering."

The sperm bank had run out of vials from Peterson's donor and could not replace them, because of restrictions health officials have instituted to protect Americans against the human form of mad cow disease. Since May 2005, the United States has effectively barred sperm banks from importing from Europe for fear it might spread the brain-ravaging pathogen that causes the affliction.

Now, as the remaining vials of Nordic semen frozen in U.S. sperm banks are running out, a small but desperate number of would-be parents are frantic. Peterson has flown repeatedly to Denmark, and went again this week, to try to get pregnant with sperm from the same donor. Others are going to Canada or Mexico, or haggling with other American women who have leftover vials.

"I think it's outrageous," said Laura, a Los Angeles lawyer who asked that her last name be withheld to protect her privacy. She decided against paying a New York woman more than $2,000 for a few vials from a donor she nicknamed "Sven," whom she used a few years ago to conceive a son. A vial usually costs less than $500. "I'd love to give him a full sibling. But I just couldn't do it. It's so unfortunate."

The restrictions on sperm from Europe were among the steps the U.S. government took in the wake of the mad cow outbreak in Europe in the late 1990s. In rare cases, people who eat meat from infected animals develop the fatal, untreatable illness called variant Creutzfeldt-Jakob disease. The ailment is caused by an infectious mutant protein that slowly eats away brain tissue. Some people have been infected through contaminated surgical equipment and transplanted tissue, such as corneas, but there are no known cases of infection from sperm.

Before the restrictions went into effect, two sperm banks -- California Cryobank in Los Angeles and Cryos International in New York City -- imported sperm from Denmark. The Nordic donors were popular because of their blue eyes and blond hair, and their tendency to be tall and have advanced degrees.

"The demand was huge," said Peter Bower of Nordic Cryobank of Copenhagen, which had supplied California Cryobank. "In addition to being tall and well educated, their motivations for donation are quite sincere -- they want to help childless couples. They tended to sell out very fast."

With California Cryobank's and Cryos's supplies virtually depleted, Nordic Cryobank filed a petition in June asking the Food and Drug Administration to lift the restrictions.

"The risk is insignificant," Bower said. "There's a huge demand, and the FDA is essentially saying to these patients they can't choose the characteristics of the children they want, even though there is absolutely no scientific evidence on their side."

Bower cites one study that concluded that getting mad cow disease from sperm is far less likely than being killed by lightning.

"They say the risk is theoretically possible, but the risk is too small and too insignificant to even be described," he said.