The human monocyte-like cell line, U937, was utilized as a model system to assess the influence of lipopolysaccharides (LPS) from Escherichia coli and other immunomodulatory agents on the biosynthesis of complement component C3 by monocytoid cells. The amount of C3 accumulated in the medium of cultured cells was measured by an enzyme-linked immunoabsorbent assay (ELISA). In the presence of LPS (0.01 and 0.10 microgram/ml) C3 production by U937 cells was increased by 2- and 5-fold, respectively. C3 production was also increased in the presence of lymphokine-containing supernatants obtained from human peripheral blood mononuclear cells after their stimulation with concanavalin A-sepharose. Human gamma-interferon (50-500 units/ml) stimulated C3 production by U937 cells. Stimulation of C3 production by LPS or mononuclear cell supernatants was blocked by cycloheximide. LPS (0.10 and 1.0 microgram/ml) also stimulated PGE2 production by U937 cells but failed to increase PGE2 at the lowest concentration (0.01 micrograms/ml) shown to increase C3 biosynthesis. Although exogenous PGE1 (10(-7) and 10(-6) M) promoted a small increase in C3 production, the effect of LPS was not decreased by a concentration of indomethacin (10(-6) M) that inhibited biosynthesis of PGE2. Therefore, LPS-induced C3 synthesis is not mediated by an increase in PGE2 in U937 cells. In summary, these observations suggest that C3 biosynthesis by monocytes/macrophages can be modulated by mediators of cellular immune responses found in the microenvironment of a localized inflammatory reaction.