Human sera from autoimmune gastritis patients containing autoantibodies to gastric parietal cells were analyzed by immunological methods. Enzyme linked immuno-sorbent assay demonstrated that all nine sera reacted with pig gastric vesicles enriched in H+/K(+)-ATPase (gastric proton pump). Immunoblotting experiments indicated that the alpha subunit of the H+/K(+)-ATPase was the major antigen in the vesicles with two of the sera reacting strongly. We further characterized the specificity of the antibodies using partial sequences of the pig alpha subunit fused with truncated TrpE (anthranilate synthase). The antibodies from autoimmune gastritis patients reacted differently to the two fusion proteins (Met-1 to Ala-79, and Arg-606 to Ile-964), indicating that each patient sera contains a mixture of autoantibodies recognizing different epitopes with variable contents.