Abstract Intraperitoneal infection with Listeria monocytogenes (LM) results in activation of the peritoneal macrophage population which displays increased surface expression of major histocompatibility (MHC) Class II (Ia) antigen and markedly suppressed prostaglandin (PG) synthesis. We demonstrate here that this decrease in PG production is also seen after treatment by mitogen (Con A) and endotoxin (LPS), and can be explained by reduced cyclooxygenase activity in these cell populations. We show that, whereas Ia expression was augmented at all doses of LM and Con A tested, it displayed a biphasic response to LPS in vivo: increase at the lowest dose and inhibition at higher doses. In order to identify possible endogenous mediators of these responses, we used highly purified preparations of recombinant murine (rMu) cytokines and neutralizing cytokine specific monoclonal antibodies (MAbs) to examine whether interferon-γ (IFN-γ) and/or tumor necrosis factor (TNF) down-regulate macrophage cyclooxygenase activity in vivo. We found that IFN-γ induced Ia expression but had no effect on PG secretion. In contrast, TNF-α suppressed PG synthesis and inhibited Ia surface expression. Similarly, in our model of Con A-induced peritoneal macrophage activation, pretreatment of animals with a neutralizing MAb to rMuIFN-γ completely blocked the induction of Ia positive macrophages by Con A but did not affect Con A-dependent suppression of PG synthesis. Pretreatment with MAb to TNF had no effect on Con A-induced Ia levels, but significantly inhibited suppressed PG synthesis. Thus, changes in macrophage cyclooxygenase activity associated with immune activation in vivo represent a manifestation of activation that is discretely modulated by TNF independently of induction of Class II expression by both Con A and IFN-γ itself.