Tumor-infiltrating lymphocytes from 120 samples of human cancers, including melanoma, renal cell carcinoma, breast cancer, sarcoma, and colon cancer, were examined. The percentage of lymphocytes recovered from the cancer varied widely; that of renal cell carcinoma was higher than that of breast or colon cancer (65% vs 45%), which was higher than that of melanomas or sarcomas (30% to 35%). The types of lymphocytes before and after interleukin 2 activation showed specific patterns. CD4+ helper T cells predominated in all tumors except melanomas, which had more CD8+ cytotoxic T cells. CD16+ natural killer cells were recovered in renal cell carcinoma and sarcomas. Three different cytotoxic lymphocytes were identified among interleukin 2-activated tumor-infiltrating lymphocytes: (1) CD3+ CD16- cytotoxic T lymphocytes with cytotoxicity restricted to autologous tumor cells in melanomas, (2) CD3-CD16+ natural killer cells with vigorous major histocompatibility complex-nonrestricted cytotoxicity in renal cell carcinoma, and (3) CD3+ CD16- T cells with modest levels of major histocompatibility complex-nonstricted cytotoxicity in all cancers except melanomas. Thus, there was considerable diversity of tumor-infiltrating lymphocytes among these histologically distinct tumors with respect to magnitude of lymphocyte infiltration, phenotypic expression, and functional capacity.