AbstractMurine peritoneal macrophages (Møs), induced with stimuli such as thioglycollate, zymosan A, OK‐432, bacille Calmette‐Guérin (BCG), or liveMycobacterium intracellulare, showed varying levels of inhibitory activity against the concanavalin A (Con A) blastogenic response of splenic T cells. All test Møs significantly inhibited the interleukin 2 (IL‐2)–producing ability of T cells but this inhibition was not enough to explain the observed reduction in T cell Con A mitogenesis. In contrast, they markedly inhibited IL‐2–reactive T cell generation, and the inhibition was sufficient to cause the reduction in T cell mitogenesis. A general relationship was observed between immunosuppressive activity of a given Mø and its active oxygen‐producing ability (measured in terms of chemiluminescence) in response to phorbol myristate acetate triggering (r= .84,P<.005). However, the suppressor activity of test Møs was not reduced by superoxide dismutase and catalase, indicating that active oxygen radicals themselves did not mediate the expression of the immunosuppressive activity of these Møs. On the other hand, indomethacin (an inhibitor of prostaglandin synthesis) caused a partial reduction in their immunosuppressive activity. The suppressor activity of Møs induced with intraperitoneal injection of recombinant interferon γ (IFN‐γ) was markedly reduced in the presence of myoglobin, a scavenger for nitric oxide radical (NO·). Tumor necrosis factor α (TNF α) failed to affect Con A mitogenesis of splenic T cells, even in combination with IFN‐γ. On the other hand, unsaturated long‐chain fatty acids including oleic, linoleic, linolenic, and arachidonic acids markedly reduced the T cell function. These findings suggest some important roles of prostaglandins, NO·, and long‐chain unsaturated fatty acids as mediators of the expression of immunosuppressive function of the peritoneal Møs.