Monoclonal antibody B72.3 recognizes a pancarcinoma antigen termed TAG-72 and is the MAb in "OncoScint CR/OV". Patients undergoing surgical resection of primary or metastatic colorectal or breast carcinoma or pseudomyxoma peritonei were injected i.v. with 125I-labeled MAb B72.3. Autoradiography identified the tissue distribution of the injected radiolabeled MAb B72.3. Immunohistochemical staining identified the corresponding spatial distribution of the target antigen, TAG-72. The labeling pattern seen using autoradiography closely matched the pattern that was observed using immunohistochemical techniques. This was especially notable in the mucin-containing compartments of the different tumors. The 125I-B72.3 was also found associated with the neoplastic cells, demonstrating a good penetration and specificity of the radiopharmaceutical through the tumor masses. The regional lymph nodes examined were the only tissues in which autoradiography and immunoperoxidase gave different results. In these specimens, the presence of TAG-72 antigen in the parafollicular area, as shown by immunoperoxidase, contrasted with the silver grain deposition, due to the 125I-B72.3, in germinal centers. These findings suggest differences in the clearance pathways of the TAG-72 antigen and B72.3 MAb.