To investigate a possible animal reservoir of Helicobacter pylori, 15 pigs, 15 rabbits and 5 cows slaughtered for consumption were studied. Raised serum IgG levels were found in 93% of the pigs and 87% of the rabbits, but levels were normal in the cattle. In the controlled testings three of 22 humans had elevated IgG to H pylori and in all three H pylori was detected by the use of monoclonal antibody. Helicobacter pylori were identified in gastric brushings by a monoclonal antibody in 8 out of 10 pigs and 7 out of 10 rabbits. This study suggests an animal reservoir of Helicobacter pylori which may be of importance in human infection.