Suppressor macrophages (M phi) which can inhibit mitogen-induced lymphocyte proliferation appeared in the spleens of mice bearing transplanted MC-A fibrosarcoma cells. An analysis of the ontogeny of such M phi revealed additional suppressor activity directed against macrophage stem cells. Treatment of spleen cell suspensions with carbonyl iron followed by centrifugation removed suppressor M phi but did not deplete M phi-colony forming cells (M-CFC) which could be demonstrated in soft agar culture in L-cell conditioned medium (LCM). Untreated spleen cells had normal numbers of M-CFC; phagocyte-depleted mononuclear cells showed a threefold increase in M-CFC 14 days after subcutaneous inoculation of 10(6) MC-A cells per mouse. Further increases in M-CFC were also evident in similar preparations on Days 21 and 28 when the M-CFC concentration reached a maximum of eight times the normal level. The M phi which developed from the M-CFC grown in the presence of LCM were later shown to have indomethacin-sensitive suppressor activity suggesting the mediation of this phenomenon by prostaglandins. These observations suggest that locally produced phagocytic suppressor M phi from the spleens of tumor-bearing mice play important roles not only as inhibitors of lymphocyte proliferation as reported earlier, but also as regulators of monocyte-M phi production.