Bone marrow macrophages (M phi) from CBA/J mice were incubated 24 h in media enriched with normal mouse serum and then stimulated with calcium ionophore A23187, phorbol ester (PMA) or zymosan for 2 more hours. Prostaglandin (PG) E2 release promoted by each agent was almost 10 times higher than from control M phi cultured without serum or with sera from other species such as rat, bovine, rabbit and human. Maximum release of PGE2 was 520 ng/mg cell protein (A23187); no release of leukotriene C4 was detectable (less than 10 ng/mg). Cellular phospholipase A2 activity was significantly enhanced by serum. These enhancing activities of mouse serum were nondialyzable, inactivated by incubation at pH 2 for 24 h, sensitive to pepsin digestion, stable at 56 degrees C for 30 min, and could not be replaced by defined cytokines such as IL-1, IL-2, IL-3, IL-4, IL-6, IFN alpha/beta, IFN gamma, TNF alpha, CSF-1, or GM-CSF. The enhancing effects of mouse serum on both PGE2 release and phospholipase A2 activity were, however, significantly blocked by exogenously added GM-CSF. In contrast with marrow M phi, thioglycollate-elicited peritoneal M phi, which normally release only small quantities of PGE2, showed over 5-fold increases in PGE2 production following treatment with IFN gamma, IFN alpha/beta, or TNF, but not with other cytokines or mouse serum. These data show stable differences in eicosanoid metabolism between M phi which suggest, in turn, highly independent regulatory mechanisms in this pathway. The data also suggest a regulatory function of GM-CSF with respect to PGE2-producing M phi formation induced by mouse sera.