The mononuclear cell infiltrate in benign and malignant breast tissue was studied using monoclonal antibodies with an immunoenzymatic technique. Infiltrating duct carcinomas contained a mononuclear cell infiltrate made up mostly of T lymphocytes, with minor B-cell and macrophage populations. The most preponderant T-cell subset was the T8, cytotoxic-suppressor cell. Some tumors contained T6-positive cells, possibly immature T cells. In contrast, stromal lymphocytes in benign fibrous mastopathy were largely T cells, but the T4, helper-inducer cell was the most preponderant, and T6 cells were not detected. Intraepithelial lymphocytes were identified in benign breast tissue. Intraepithelial lymphocytes are T cells, with a great preponderance of the T8 phenotype. The T6-positive cells were also identified within the epithelium, but they seemed to represent myoepithelial cells rather than immature T cells.