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The nature of host-derived, specifically cytotoxic cells infiltrating solid murine mammary adenocarcinomas (line T1699) was investigated. Cell suspensions obtained from enzymatically dispersed tumors were separated by sedimentation velocity. The host cell fraction was heterogeneous and contained T-lymphocytes, non-phagocytic cells bearing Fc receptors, eosinophils and monocytes. Host cells equivalent in size to small lymphocytes and bearing Fc receptors were found to be the predominant cell type responsible for colony inhibition. These colony-inhibiting cells were insensitive to lysis with either anti-theta or anti-IgG serum and complement; and they were specific both in their induction and in their mediation and in their mediation of cytotoxicity. Host cells made up the predominant population in spontaneously regressive tumors but only a minor component in progressive tumors, and they were totally absent from progressive tumors in X-irradiated animals.

Morphologic studies of two unrelated progressively growing sarcomas in rats revealed the presence of a large proportion of host cells distinct from tumor cells. Enzymatically or mechanically dispersed tumor suspensions were fractionated by the velocity sedimentation method, which resulted in a separation of host and tumor cells. The host cells had the typical morphology of macrophages and also were adherent to a plastic surface after a short incubation period. The proportion of host cells was greater in a young (10 or 12 days) than in an older tumor (35 days). These macrophage-like host cells isolated from sarcomas 12 and 35 days after implantation inhibited the colony formation of tumor cells in vitro in the nonspecific manner ascribed to activated marcrophages. In addition to these effector cells, the host fraction contained macrophage precursors able to proliferate in vitro but apparently inhibited by the presence of tumor cells. The results indicated that these macrophage-like effector cells may be the predominant host reaction within some rat sarcomas, and also that thymus- and bone marrow-derived lymphocytes act only indirectly if they are involved.