“…Nonadherent, nonphagocytic radio-resistant "natural killer" (NK) cells, distinct from mature T and B cells, and found predominantly in the peripheral blood and spleen of rodents and man, have selective cytotoxicity for a limited range of target cells (Haller et al, 1977;Herberman et al, 1975: Kiessling et al, 1975Shellam, 1977;Shellam and Hogg, 1977;Nunn, Herberman and Holden, 1978). In mice, adherent nonphagocytic peritoneal cells exhibiting spontaneous antitumour cytotoxicity have been reported (Nathan, Hill and Terry, 1976) probably representing a subpopulation of B lymphocytes (Nathan, Asofsky and Terry, 1977 Target cells Early passages of DA rat embryonic fibroblasts (Keller, 1976b), DA rat dimethylbenz-(a)anthracene-induced ascites tumour cells (Keller, 1977a), polyoma-virus-induced tumours (Keller, 1973), early passages of epidermal cells from the skin of normal BALB/c mice (Keller, 1977h), DBA/2 murine mastocytoma P815 (Keller, 1976b) SY740-transformed mouse macrophages (Keller, 1977b) RPMI 7932 human melanoma cells (Keller, 1976b) and the Burkitt's lymphoma cell line RAJI (Keller, 1976b) (Keller, 1974(Keller, , 1976a FCS. After appropriate-macrophage-targetcell interaction, radioactivity was measured in sediments and supernatants as described by Keller (1976c), and the cytotoxicity calculated by the following formula:…”