Summary Human blood monocytes were cytotoxic towards cells of a human osteosarcoma and a reticulum cell sarcoma in vitro. Activation of the monocytes by supernatant fluids of mitogen stimulated human lymphocytes markedly increased their cytotoxicity towards the neoplastic cells. Intradermal injection of these supernatant fluids into human skin induced an inflammatory infiltrate consisting predominantly of large mononuclear cells. The role of large mononuclear cells in cancer surveillance is discussed.