The authors examined inflammatory mononuclear cells in 10 fibroadenomas and 56 ductal infiltrating type carcinomas of the breast to see whether the distribution of various subpopulations of the mononuclear cells were correlated with known histologic, biochemical, and clinical parameters of the cancers. T cells, B cells, natural killer cells, and macrophages were quantitated on frozen tissue sections, which were stained with monoclonal antibodies, as demonstrated by the immunoperoxidase technique. The carcinomas had significantly higher numbers of T cells, Leu-3+ helper-inducer cells, T8+ cytotoxic-suppressor cells, M5+ macrophages, and T6+ Langerhans cells than the fibroadenomas. There were no significant differences in the distribution of natural killer cells. Among these mononuclear subsets, helper-inducer T cells were the primary cell type in both benign and malignant neoplasms. Estrogen receptor-positive carcinoma had significantly fewer numbers of T cells and the subgroup of Leu-3+ helper-inducer cells. Clinical Stage 3 cancers had significantly fewer numbers of T cells and natural killer cells than both fibroadenomas and Stage I and II neoplasms. Prominent infiltration of T cells, specifically helper-inducer cells, is not associated with estrogen receptor positivity, which is known to be a favorable prognostic marker of breast carcinoma. The findings of fewer T cells and natural killer cells in Stage 3 carcinomas are intriguing but unexplained.