Breast carcinomas were examined by the immunoperoxidase technique using antisera specific for lymphocyte subsets, monocytes, NK cells and major histocompatibility antigens (HLA-A, -B, -C; Ia-like). Sixty-four per cent of the patients had a moderate or strong mononuclear cell infiltration, 77% of the patients without mononuclear cell infiltration had receptors for estrogens as compared to 51% of the patients with infiltration. The majority of the infiltrating mononuclear cells were T cells; generally the OKT8 cells were predominant. The Leu 3A/OKT8 cell ratio was not related to histological type, tumor size, age of the patient or presence of metastases. Some of the T cells had the Ia antigen and were thus probably activated. The B cells were either absent or less numerous than the T cells. There was no relation between their distribution and the various parameters studied. A few monocytes were heterogeneous according to their markers (OKM I and acid phosphatase). In 6 cases only there was a strong infiltration of mononuclear cells positive for acid phosphatase. The number of the natural killer cells was also low. Only a few mononuclear infiltrating cells had receptors for transferrin. There was a positive correlation between the inflammatory infiltration and the presence of HLA class-I antigens on tumor cells. Some of the antisera specific for lymphocyte subsets also stained the breast carcinoma cells. The great variations in the subsets of mononuclear cells in breast carcinomas may correspond to various systems of defense against neoplasm.