We examined the possibility that the anti-estrogens, tamoxifen (TX) and toremifen (TO) interacted with the immune system. Indeed, both TX and TO stimulated cells mediated cytotoxicity reactions by various killer cells: killer T (TK), natural killer (NK), lymphokine activated killer (LAK) cells. Both TX and TO inhibited the growth of tumors that express estrogen receptors. Thus these antiestrogens inhibited tumor growth and stimulated killer cells for cytotoxicty on such tumors. Therefore these agents were presumed to stimulate tumor immunity. We tested the P815 mouse mastcytoma with TK, LK, and TX or TO. A therapeutic effect was observed in both experiments. The SL2-5 murine lymphoma was tested with NK and TX cells or TO cells and successful immunotherapy was observed. We digested human breast carcinomas and lung tumors with collagenase. The killer cells were separated from tumor cells on Ficoll gradients. TX and TO enhanced the cytotoxic effect of autologous killer cells on the corresponding tumor cells. This experiment indicates that the results obtained in animals are also valid for human malignant disease.