Splenocytes from old rats exhibit a significant reduction in proliferative responses to Con A. This defect can be overcome by passing the cells over nylon wool, which suggests that nylon wool adherent cells contribute to the reduction in proliferation of mixed cell populations from aged rats. To examine the potential role of macrophages in this process, thioglycollate-elicited peritoneal macrophages were added to Con A-activated spleen cells from young and old rats. Addition of as few as 3% macrophages suppressed proliferation of Con-A-activated splenocytes and this inhibition was significantly greater if either the macrophages or spleen cells were derived from aged rats. Although prostaglandin E2 (PG-E2) inhibited the proliferation of splenocytes and nylon-wool enriched splenocytes from 3-month-old animals, its suppressive effects were much greater on Con A-activated lymphocytes derived from 24-month-old rats. Addition of catalase to deplete macrophage-derived H2O2 resulted in greater augmentation of Con-A-induced proliferative responses of splenocytes from old rats than those from young rats. TGF-beta 2, but not TNF-alpha, also significantly suppressed the proliferation of Con A-activated splenocytes to a greater degree in aged than in young rats. Since macrophages are known to release PG-E2, H2O2 and TGF-beta 2 after stimulation with Con A, these data support the idea that T cells within the splenocyte population become increasingly susceptible during aging to inhibitory products released by macrophages.