Monoclonal antibody (MAb) B72.3 reacts with a tumor-associated glycoprotein, designated TAG-72. TAG-72 has been identified in many human carcinomas but generally is not found in normal human tissues. Because of its proven utility in the diagnosis of human carcinomas, MAb B72.3 was applied to several different types of canine carcinomas. Five types of formalin-fixed, paraffin-embedded canine carcinomas were evaluated for immunoreactivity with MAb B72.3 by use of an avidin biotin immunoperoxidase complex method. Samples were considered positive when > or = 5% of all malignant cells contained a distinct intracellular stain. Immunoreactivity for MAb B72.3 was observed in 6/9 (67%) pulmonary adenocarcinomas, 7/13 (54%) transitional cell carcinomas, 7/11 (64%) mammary adenocarcinomas, 7/11 (64%) nasal adenocarcinomas, and 1/2 (50%) prostatic adenocarcinomas. The average cellular staining for positive carcinomas was 25%. Normal canine tissues from similar anatomic sites had little or no individual cell immunoreactivity. These preliminary results indicate that some canine carcinomas may express a tumor-associated antigen that is similar to TAG-72 and that MAb B72.3 immunoreactivity may be of diagnostic significance in classifying animal tumors.