Hog gastric vesicles showed Cl- conductance when treated with Cu2+-o-phenanthroline, an S-S cross-linking reagent. An IgG monoclonal antibody caused dose-dependent inhibition of Cl- conductance that had been induced by S-S cross-linking. The antibody did not cause intervesicular aggregation, as determined by measurement of vesicle size. These results show that Cl- conductance, the stimulation and inhibition of which are regulated reversibly by S-S----2SH transformation, is due to native, physiological channels. The antibody also dose dependently inhibited the activities of H,K-ATPase and p-nitrophenyl phosphatase in gastric vesicles, but did not inhibit Na,K-ATPase obtained from dog kidney. Immunoblotting with the antibody of vesicle proteins solubilized in sodium dodecyl sulfate-polyacrylamide gel showed that the antibody binds to a 95-kDa subunit of H,K-ATPase and its dimeric 180-kDa polypeptide. The antibody-binding sites of H,K-ATPase activity and the Cl- channel for the inhibition were present on the external (cytosolic) surface of the transmembraneous ATPase. A gastric antisecretory compound, 2-methyl-8-(phenylmethoxy)imidazo[1,2 alpha] pyridine-3-acetonitrile (SCH 28080), competitively bound to the high affinity site of K+ on the internal (luminal) surface of H,K-ATPase, and its half-maximal inhibitory concentration for H,K-ATPase activity in tight vesicles was 0.2 microM in the presence of valinomycin. SCH 28080 also dose dependently inhibited opening of Cl- channels by S-S cross-linking, the regulatory site being present on the cytosolic side and more internally than the antibody binding site. The half-inhibitory concentration of SCH 28080 was 0.3 microM. The present results with the antibody and SCH 28080 indicate that the Cl- channel is part of the function of H,K-ATPase.