Polyclonal antibodies to the purified mouse erythrocyte anion exchange protein (band 3) and to a conserved COOH-terminal peptide of mouse band 3 (alpha-Ct) recognized a single major 185-kDa polypeptide in immunoblots of a membrane fraction prepared from rabbit gastric glands. Competition studies revealed that the epitopes shared between the rabbit gastric 185-kDa antigen and the approximately 100-kDa mouse erythrocyte band 3 protein are restricted to the COOH-terminal domain of band 3, which is known to contain the catalytic site for anion exchange activity. Immunofluorescence microscopy was used to demonstrate that this band 3-related polypeptide is associated with the plasma membrane in a subpopulation of gastric gland cells composed exclusively of oxyntic cells, as judged by the coincidence of immunofluorescence with alpha-Ct and with a monoclonal antibody to the gastric H+-K+-ATPase. This alpha-Ct-reactive antigen was further localized to the cytoplasmic face of the basolateral membrane of oxyntic cells, which correlates well with the physiologically determined site of anion exchange activity. These data demonstrate the presence in gastric oxyntic cells of a novel member of the family of proteins related to the erythrocyte anion exchanger. The possibility that the 185-kDa polypeptide is an anion exchanger is discussed.