Using an antibody raised against highly purified dog renal proximal tubule brush border membranes, we have investigated the tissue distribution and ontogenic development of antigens sharing specificity with those in the renal brush border membrane. The antigens have been found in the small intestine, gall bladder, epididymis, pancreas, salivary gland, lacrimal gland, liver and spleen. Examination of embryos at various stages of gestation has revealed that the antigens make their appearance early in organogenesis, and are present in the cytoplasm as well as on the membrane during foetal development. Absorption studies have shown that at least two populations of antigens are present in the kidney: one appears to be kidney-specific and the other is shared by many epithelia. Maltase has been identified as one of the shared antigens.