The macrophage is a major component of the inflammatory response induced by lymphatic tissue-dwelling filariae. Intraperitoneal (i.p.) infections with Brugia pahangi in Mongolian gerbils, or jirds (Meriones unguiculatus), induce a peritoneal inflammatory response characterized by accumulation of numerous macrophages and fewer eosinophils. This inflammatory response is associated with the release of microfilariae by female worms. The aim of this study was to investigate the activation state of the peritoneal macrophages during the course of i.p. infections with either male or female worms. Activation was determined by a toxoplasmacidal assay and assays which measured the production of tumor necrosis factor (TNF)-like activity and nitric oxide (NO) production. The development of these assays with jirds was initially conducted in parallel with the mouse system, which served as a positive control. Jird macrophages became activated to kill Toxoplasma gondii by in vivo immunization with Mycobacterium bovis BCG in a pattern similar to that of mouse macrophages. However, unlike the mouse system, supernatants from purified protein derivative- or concanavalin A-stimulated jird splenocytes plus lipopolysaccharide failed to activate jird macrophages in vitro or induce NO production. These results indicate that factors involved in jird macrophage activation may differ from those demonstrated in the mouse system and other systems. i.p. infections of 15 days in duration with either male or female worms induced macrophage activation as measured by Toxoplasma killing and TNF production. These responses decreased as the infection progressed to the chronic period on a time course that parallels the down regulation of experimental B. pahangi granulomas. There was no evidence of NO production by activated jird macrophages. These data indicate that macrophage function is down modulated during filarial infection and suggest that mechanisms involved in macrophage deactivation are related to those that induce down modulation of the systemic granulomatous inflammatory response in the jird. This response is not dependent on the microfilarial stage of the parasite and is also independent of mechanisms which induce peritoneal accumulations of macrophages.