The role of macrophages in tumor-mediated immunosuppression was examined, using C57B1/6 strain mice bearing four different immunosuppressive transplantable syngeneic tumors (Lewis Lung Carcinoma, B16 Melanoma, and two fibrosarcomas induced by methylcholanthrene in our laboratory). When tested for immunosuppressive activity, in inhibiting the induction of antibody formation by normal spleen cells in response to SRBC in vitro, the splenic and peritoneal macrophages from tumor-bearing mice were all significantly suppressive. The degree of suppression correlated with immunosuppression in tumor-bearing mice challenged in vivo with SRBC. Direct action of tumor cells on normal splenic macrophages in vitro caused them to become suppressive, the extent of suppression dependent on the time of interaction and on the immunosuppressive activity of the tumor cells in vivo. Pretreatment of suppressive splenic macrophages with indomethacin, a potent inhibitor of the synthesis of prostaglandins (PG), reduced significantly their immunosuppressive activity. Also, peritoneal macrophages from tumor-bearing mice produced significantly more PGE in culture than control macrophages. Thus, tumor-activated macrophages, presumably those macrophages that infiltrate the tumor in a host reaction against the tumor, serve to amplify the level of immunosuppression in the host by producing relatively large amounts of PGE that is a key physiological mediator in the activation and function of suppressor T lymphocytes. The stimulation of PGE synthesis in macrophages, as a result of their interaction with syngeneic tumors, is initiated by PGE produced in relatively large amount by the tumor cells.