Biologic properties of breast cancer in men that might reflect alterations in pathogenesis from the disease in women were examined. We studied 22 tumors from males, 18 invasive carcinomas, three of which were papillary, and three in situ tumors of which one was papillary, and one papilloma. Our data support the previously reported high incidence of papillary carcinoma in men. Estrogen receptor status and the expression of cancer-associated antigens recognized by antibodies DF3, B73.2, SP-1, and c-erbB-2 were compared to matched tumors from females. Immunocytochemistry was performed on formalin-fixed, paraffin-embedded sections using standard avidin-biotin techniques; anti-PSA was used to exclude the possibility of metatastic prostate cancer, and 12 cases of gynecomastia were included as nonmalignant controls. The incidence of estrogen receptor positivity was higher in tumors from males (73%) than from females (54%), as has been reported previously. The range of expression of all breast cancer antigens tested in male tumors was similar to that observed in females, but some interesting differences were noted. With the exception of the anti-mucin DF3, all the antibodies reacted only with neoplastic tissues. Expression of the oncoprotein c-erbB-2 was lower (17%) in males than in females (33%), despite the preponderance in men of the large-cell type carcinomas that have been associated with c-erbB-2 expression. Unexpectedly, the pregnancy-associated hormone detected by SP-1 was expressed in 33% of tumors from males and, in contrast to females, was found in less differentiated tumors.