To investigate a potential new treatment for gastric Helicobacter pylori infection, we have examined the use of the natural antibiotic lactoferrin, found in bovine milk, for activity against Helicobacter species both in vitro and in vivo. Lactoferrin was bacteriostatic to H. pylori when cultured at concentrations > or =0.5 mg/ml. Growth of H. pylori was not inhibited by another milk constituent, lysozyme, or by a metabolite of lactoferrin, lactoferricin B, but growth was inhibited by the iron chelator deferoxamine mesylate. Lactoferrin inhibition of growth could be reversed by addition of excess iron to the medium. Lactoferrin in retail dairy milk was found to be more stable intragastrically than unbuffered, purified lactoferrin. Treatment of H. felis-infected mice with lactoferrin partially reversed mucosal disease manifestations. It is concluded that bovine lactoferrin has significant antimicrobial activity against Helicobacter species in vitro and in vivo. Bovine lactoferrin should be further investigated for possible use in H. pylori infections in man.