Lymphocytes from patients who had fibroadenoma alone or combined with fibrocystic disease of the breast were specifically cytotoxic to breast adenocarcinoma cells, as measured in vitro by the microcytotoxicity test. The cell-mediated immunity was specific for breast tumor cells and there was no unexpected cross-reactivity with other types of tumor cells or with normal fibroblasts. In some cases, criss-cross type experiments were used between fibroadenoma patients and those with unrelated types of cancer. These results implied the existence of a common antigen on the cell surface of fibroadenomas and adenocarcinomas of the breast.