Variability of tumor-associated antigens among and within human tumor cell groups presents a potential problem in the development and optimization of immunodiagnostic and therapeutic procedures for cancer. We determined the degree of expression of a tumor-associated antigen in the primary and metastatic lesions of 23 patients with infiltrating ductal carcinoma; this was accomplished using monoclonal antibody B72.3, an IgG1 generated against membrane-enriched fractions of human metastatic breast carcinomas and reactive with a 220,000-400,000 d glycoprotein complex, termed TAG-72, and the avidin-biotin complex immunoperoxidase method on fixed tissue sections. Sixteen of the 23 breast carcinomas (70%) demonstrated MAb B72.3 reactivity (range 5% to 100% of tumor cells staining). Reactivity of lymph node metastases was present in 14 of 21 patients (67%). MAb reactivity in metastases to distant sites, including bone, adrenals, liver, skin and effusions, was present in 10 of 18 patients (56%). In one patient, neither the primary carcinoma nor the metastasis to the lymph node demonstrated reactivity. There was a statistically significant positive correlation between MAb B72.3 reactivity in both primary and lymph node metastases (Kendall's Correlation Coefficient = 0.60, p = 0.0006) and between lymph node and distant metastases (Kendall's Correlation Coefficient = 0.48, p = 0.02) of the same patient. No correlation existed between antibody reactivity seen in the primary and that found in the distant lesions of that patient. These studies thus demonstrate that monoclonal antibody B72.3 can detect expression of a tumor-associated antigen in both primary and metastatic infiltrating ductal carcinoma lesions, and may prove valuable in the understanding of tumor biology of metastases and as a means for diagnosing occult disease.