Monsanto Co., one of the giants in agribusiness, has promised not to experiment with biotechnology that leaves seeds sterile--a possibility that has raised fears among critics of gene manipulation.

Monsanto Chairman Robert B. Shapiro said concerns have made it important for him to stress his commitment against the practice of manipulating three genes to make a seed good for only one planting cycle.

His comments were contained in a letter sent to Gordon Conway, president of the Rockefeller Foundation. Though noting that gene-altered crops can help reduce world hunger, Shapiro said he opposes the idea of sterile seeds. On Monday, Conway called Monsanto's decision a first step in making "the fruits of plant biotechnology" available to poor farmers worldwide.

Producing crops with infertile seeds could prove very valuable to agribusinesses, which already make genetically altered seeds that are resistant to insects and herbicides. Opponents fear companies could begin offering those traits only in sterile seeds, which would require farmers to buy them each year rather than replanting seeds gathered from mature crops.

Monsanto has been criticized over the issue even though it hasn't done any research on making sterile seeds. The technology was developed by the U.S. Department of Agriculture and Delta & Pine Land Co., the world's largest cotton seed company. It was patented in 1998, two months before Monsanto offered to buy Delta--a bid awaiting an antitrust review by the Justice Department.

Delta said Monday that it would continue trying to commercialize the seed sterilization technology.