Four murine monocyte, myelomonocyte and histiocyte or macrophage tumor cell lines adapted to culture were growth inhibited by 1 or more of the following macrophage activating substances: Mycobaterium bovis, BCG strain, zymosan, [Salmonella typhosa] lipopolysaccharide and dextran sulfate, as well as tuberculin purified protein derivative, but not latex beads. Lipopolysaccharide was effective with 1 line at 4 ng/ml. All 4 lines actively phagocytosed zymosan and latex beads. In many cases the growth inhibition was apparently immediate but only cytostatic, and cell proliferation resumed on removal of the drug. BCG, live or boiled, was toxic to some of the tumor lines. Synthesis of lysozyme by all cell lines in the monocyte series and production of granulocyte colony stimulating factor by the myelomonocyte leukemia were not inhibited during several days of zero growth conditions in the presence of drugs. Since these agents had no direct effect on other hematopoietic tumor types (myeloma, T[thymic]-lymphoma, mastocytoma) at the same or up to 104 higher concentrations, the sensitive tumors retina specific receptors for immunostimulants, at the cell surface or within the cell in the case of phagocytosable particles. The binding of these agents to physiological receptors leads to stimulation and mitogenesis in normal macrophages and lymphocytes but leads to growth inhibition without affecting differentiated functions in the corresponding tumor lines.