With his mere 300 acres of soybeans, corn and wheat, Vernon Hugh Bowman said, “I’m not even big enough to be called a farmer.”

Yet the 75-year-old farmer from southwestern Indiana will face off against the world’s largest seed company, Monsanto, in a Supreme Court case that could have a huge impact on the future of genetically modified crops, and also affect other fields from medical research to software.

At stake in Mr. Bowman’s case is whether patents on seeds — or other things that can self-replicate — extend beyond the first generation of the products.

It is one of two cases before the Supreme Court related to the patenting of living organisms, a practice that has helped give rise to the biotechnology industry but which critics have long considered immoral. The other case, involving a breast cancer risk test from Myriad Genetics, will determine whether human genes can be patented. It is scheduled to be heard April 15.

Monsanto says that a victory for Mr. Bowman would allow farmers to essentially save seeds from one year’s crop to plant the next year, eviscerating patent protection. In Mr. Bowman’s part of Indiana, it says, a single acre of soybeans can produce enough seeds to plant 26 acres the next year.

Such a ruling would “devastate innovation in biotechnology,” the company wrote in its brief. “Investors are unlikely to make such investments if they cannot prevent purchasers of living organisms containing their invention from using them to produce unlimited copies.”

The decision might also apply to live vaccines, cell lines and DNA used for research or medical treatment, and some types of nanotechnology.

Many organizations have filed briefs in support of Monsanto’s position — universities worried about incentives for research, makers of laboratory instruments and some big farmer groups like the American Soybean Association, which say seed patents have spurred crop improvements. The Justice Department is also supporting Monsanto’s argument.

BSA/The Software Alliance, which represents companies like Apple and Microsoft, said in a brief that a decision against Monsanto might “facilitate software piracy on a broad scale” because software can be easily replicated. But it also said that a decision that goes too far the other way could make nuisance software patent infringement lawsuits too easy to file.

Some critics of biotechnology say that a victory for Mr. Bowman could weaken what they see as a stranglehold that Monsanto and some other big biotech companies have over farmers, which they say has led to rising seed prices and the lack of high-yielding varieties that are not genetically engineered.

Patents have “given seed companies enormous power, and it’s come at the detriment of farmers,” said Bill Freese, science policy analyst for the Center for Food Safety, which was an author of a brief on the side of Mr. Bowman. “Seed-saving would act as a much needed restraint on skyrocketing biotech seed prices.”

Farmers who plant seeds with Monsanto’s technology must sign an agreement not to save the seeds, which means they must buy new seeds every year.

Monsanto has a reputation for vigorously protecting its intellectual property.

The Center for Food Safety, which has tracked the cases, said Monsanto had filed more than 140 patent infringement lawsuits involving 410 farmers and 56 small farm businesses, and had so far received $23.67 million in recorded judgments. The center says there are many other cases in which farmers settled out of court or before a suit was filed.

Monsanto says it must stop infringers to be fair to the large majority of farmers who do pay to use its technology.

But Monsanto typically exercises no control over soybeans or corn once farmers sell their harvested crops to grain elevators, which in turn sell them for animal feed, food processing or industrial use.

Mr. Bowman said that for his main soybean crop, he honored Monsanto’s agreement, buying new seeds each year containing the Roundup Ready gene, which makes the plants immune to the herbicide Roundup. That technology is hugely popular, used in more than 90 percent of the nation’s soybeans, because it allows farmers to spray fields to kill weeds without hurting the crop.

But Mr. Bowman also planted a second soybean crop later in the growing season, after harvesting his wheat. Since that late crop is more prone to failure, he says, he did not want to pay the high price for patented seed.

So starting in 1999, he bought commodity soybeans from a grain elevator. These beans were a mixture of varieties from different farmers, but, not surprisingly, most of them were Roundup Ready. So Mr. Bowman sprayed Roundup on his late-season crop.

“All through history we have always been allowed to go to an elevator and buy commodity grain and plant it,” he said in an interview.

The courts, however, have not agreed. After Monsanto sued Mr. Bowman in 2007, a district court in Indiana awarded the company more than $84,000. The Court of Appeals for the Federal Circuit, which specializes in patent cases, upheld that decision, saying that by planting the seeds Mr. Bowman had created newly infringing articles.

The Supreme Court, which has generally been taking a narrower view of patent rights than the appellate court, agreed to hear the case, much to the chagrin of the biotechnology industry.

“We did not see it coming,” said Hans Sauer, deputy general counsel of the Biotechnology Industry Organization. “We thought this question was long decided.”

Despite the arguments for and against genetically modified crops contained in various briefs, the decision will probably rest on the intricacies of patent law.

Mr. Bowman’s main defense is patent exhaustion — the concept that once a patented object is sold, the patent holder loses control over how it is used.

The Supreme Court affirmed this principle most recently in a 2008 case involving Intel computer chips containing patented technology licensed from LG Electronics. The court ruled that once Intel sold the chips to computer manufacturers, LG’s rights were exhausted and LG could not control how the manufacturers used the chips in their machines.

In the seed case, Mr. Bowman argues, Monsanto had no more rights on the beans sold to the grain elevator.

Monsanto and its allies counter, saying that selling to a grain elevator and buying back seed would be too simple an end-run around the patents. They say exhaustion applies only to the particular item sold and does not give the buyer the right to make unlimited copies of that item, which is what Mr. Bowman did by growing new soybean plants from the grain elevator beans.

Mr. Bowman’s side says that he was not making seeds, he was merely using the seed from the elevator, which is permissible. It just so happens that because of the nature of a seed, using one results in more copies being created. Mr. Bowman argues there should not be an exception to the exhaustion rule just because a patented item is self-replicating.

Mr. Bowman and his allies say a ruling in his favor would not decimate the biotechnology industry. Monsanto could still use contracts to stop farmers from saving newly purchased seed, which is what most farmers will continue to use. Most farmers would not buy large amounts of seed from grain elevators, they say, because it is of poor quality.

Mr. Bowman said that before his case, Vernon Hugh Bowman v. Monsanto, 11-796, was taken pro bono by Mark P. Walters and other lawyers from the firm of Frommer Lawrence & Haug, he had spent $31,000 on legal fees and handled much of the legal research himself, using a computer at the library because he does not own one. He said he never considered settling because he thought he was in the right.

“I was prepared to let them run over me,” Mr. Bowman said, “but I wasn’t getting out of the road.”