By screening microbial extracts for products that stimulate innate host defense mechanisms, scientists at Magainin Pharmaceuticals (Plymouth Meeting, PA) have determined that l-isoleucine and several of its analogs can induce the expression of β-defensins (PNAS 97, 12723–12728, 2000). Isoleucine, which is nontoxic and can be given as a food additive, is now being tested as an alternative to antibiotics in farm animals. Defensins, bactericidal proteins secreted by mucosal epithelial cells, have been studied extensively as potential tools for combating bacterial infections, but the new work demonstrates that defensin production can be induced in cultured cells using isoleucine, an essential amino acid in the human diet that has already been proven safe. Indeed, senior author Michael Zasloff says that “with several large animal feed producers we have been evaluating the utility of isoleucine as a feed additive. . .in the poultry industry.” The amino acid may also be incorporated into a new antimicrobial vaginal gel being developed by the company. In both cases, the researchers hope that inducing defensins prophylactically will prevent pathogenic bacteria from becoming established in the host, reducing the need for subsequent antibiotic treatment.