Analysis by indirect immunoperoxidase staining of tissues demonstrated that monoclonal antibody (mAb) K2.7 derived from a mouse immunized with a renal cancer cell line OS-RC-2 was reactive with the tissues of all 36 normal kidneys examined, including two fetal kidneys, 22 of 25 renal cell carcinomas (RCCs) and one of 15 gastric cancers, but not with 23 normal tissues from 11 different organs or 41 malignant tissues from 10 different organs. mAb K2.7 was reactive with the kidneys of several animal species. Positive staining was seen on tubular epithelial cells of the kidney. Thus, the antigen recognized by mAb K2.7 appeared to be an interspecies kidney specific antigen. Results obtained by protein A (PA) assays with a cultured cell panel were consistent with those of staining and confirmed the restricted expression of the antigen. Immunochemical analysis revealed that mAb K2.7 reacted with molecules of 63,000, 60,000 and 43,000 daltons.