Abstract Host lymphoid cells infiltrating two 3-methylcholanthrene-induced sarcomas transplanted in syngeneic Wistar rats were isolated and tested for cytolytic reactivity in vitro against cultured tumor targets. In 6 hr 51Cr release tests, lymphoid cells from both tumors showed direct cytotoxicity against a number of in vitro derived tumor targets. This activity, although quantitatively usually less than that of normal or tumor-bearer spleens, had the characteristics of splenic natural killer (NK) cells, since the effectors were nonadherent, nonphagocytic, and showed the same target cell specificities as splenic NK cells against a panel of nine tumor cell lines. No evidence of specific cytotoxicity directed against the tumor of origin was found. The demonstration of NK cells within tumors provides further, albeit circumstantial, evidence for a possible role for these cells in the host response to solid tumors in vivo.