A legal ruling says contracts give Monsanto the right to stop rival DuPont from selling genetically modified soybeans built with Monsanto's technology, but leaves open DuPont's challenge of the restrictions on antitrust grounds.

The ruling in St. Louis federal court is the latest turn in a lawsuit between the world's two biggest seed companies. At issue is how much freedom Monsanto Co.'s competitors have to develop crops containing their own biotech traits using Monsanto's patented Roundup Ready gene, which is inserted in the vast majority of U.S. corn and soybean crops.

Monsanto sued DuPont last spring, claiming it was illegal for DuPont to sell its new line of biotech seeds called Optimum GAT. That line of seeds add a new DuPont gene to the older line of Roundup Ready corn and soybean plants that DuPont developed under a license with Monsanto.

U.S. District Judge Richard Webber said in Friday's ruling that Monsanto's licensing agreement clearly prohibits DuPont from inserting its Optimum GAT gene into corn and soybean plants with Monsanto traits.

But Webber said his ruling was narrow, and didn't consider whether Monsanto has the right under antitrust laws to restrict how competitors breed and sell plants with Monsanto traits.

DuPont is challenging its licensing agreement with Monsanto on antitrust grounds, in the midst of a U.S. Department of Justice antitrust investigation into Monsanto that is examining whether there is anticompetitive behavior in the seed industry.

"This litigation is just beginning; we will now vigorously pursue our antitrust, license and patent fraud claims," DuPont Senior Vice President and General Counsel Thomas L. Sager said in a statement Saturday.

Monsanto was not immediately available for comment.

Monsanto announced this week that the Justice Department demanded internal documents related to the company's soybean business. Spokesman Lee Quarles said the company has done nothing wrong and is cooperating with the department, providing millions of pages of documents it requested.