Human calcium transporting tissues were examined to determine whether they contained a protein similar to the Ca++-Mg++ adenosine triphosphatase (Ca++-Mg++ATPase) pump of the human erythrocyte membrane. Tissues were processed for immunoperoxidase staining using monoclonal antibodies against purified Ca++-Mg++ATPase. In human kidneys, specific staining was found only along the basolateral membrane of the distal convoluted tubules. Glomeruli and other segments of the nephron did not stain. Staining of erythrocytes in human spleen was readily observed. Human small intestine, human parathyroid, and human liver showed no antigens that crossreacted with the antibodies to Ca++-Mg++ATPase. Specific staining of distal tubule basolateral membranes from the kidney of a chimpanzee was also noted. Our experiments show, for the first time, that basolateral membranes of the human distal convoluted tubule contain a protein that is immunologically similar to the human erythrocyte Ca++-Mg++ATPase. These observations suggest that the cells of the distal convoluted tubules of human kidney may have a calcium pump similar to that of human erythrocyte membranes.