Medullary carcinoma of the breast, which is usually associated with a dense lymphocytic infiltrate, carries a better prognosis than do most other histologic subtypes of breast carcinoma. We studied cryostat-cut fresh frozen sections from 12 patients with medullary carcinoma and, as controls, nine patients with infiltrating ductal carcinoma in order to determine and compare the antigenic phenotype of the lymphocytic components of these tumors. We used a large panel of monoclonal antibodies and polyclonal antisera for T-cells (Leu-1, Leu-2a, Leu-3a, Leu-9, T-3, T-6, T-10, T-11, and TQ-1), pre-B and B-cells (BA-1, B-1, B-2, B-4, and J5), NK cells (Leu-7 and Leu-11b), and cell activation associated antigens (T-9, HLA-Dr, and Tac). The most commonly encountered antigens on the lymphocytic components of both medullary carcinoma and infiltrating ductal carcinoma were: T-3, T-11, Leu-1, Leu-2a, Leu-3a, and Leu-9. There was little staining for NK-, pre-B-, or B-cell associated antigens in either type of carcinoma. However, the lymphocytes in the control cases tended to express HLA-Dr and T-10 more often than did the lymphocytes in the cases of medullary breast carcinoma. Our data indicate that: the antigenic phenotypes of the lymphocytic infiltrates of medullary carcinoma and those of infiltrating ductal carcinoma of the breast are essentially similar; and the lymphocytes in these carcinomas are composed predominantly of peripheral T-lymphocytes. We therefore conclude that the favorable biologic behavior of medullary carcinoma of the breast cannot readily be explained by the immunophenotype of its lymphocytic component.