An antigen immunologically related to mouse mammary tumor virus (MuMTV) and the major envelope glycoprotein, gp52 of MuMTV, was identified in tissue sections of human male and female mammary carcinomas using the peroxidase-antiperoxidase technique. The specificity of the reaction was established by absorption studies. Positive reactions with the gp52 antiserum were seen in mouse and human mammary carcinomas, but not in normal mammary tissues, mammary tissues with benign diseases and in other primary malignant neoplasms. Almost all (32/36, 89%) male mammary carcinomas were positive for the gp52 related antigen. A lesser proportion of tumors among female patients (14/50, 28%) were positive. The gp52 positive tumors were significantly larger than the gp52 negative tumors in female patients (P less than 0.05). Gp52 positive tumors were also larger than gp52 negative tumors in male patients, but the difference was not statistically significant. Gp52 reactivity was also detected in metastatic mammary carcinoma in axillary lymph nodes of male and female patients. The presence of gp52 related antigen was not apparently related to tumor grade or lymphocytic infiltrate in the primary tumor. The data do not permit a firm conclusion regarding nodal status in men; no correlation of gp52 activity and nodal status in women was evident. These results indicate that mammary carcinomas in men as well as in women have an antigen related immunologically to MuMTV gp52. Other than tumor size, the antigen seems to be unrelated to major prognostic factors. The significance of the antigen with respect to etiologic features and prognosis in breast cancer remains to be determined.