We have studied the effect of IgA- and IgG-containing immune complexes on the production of complement proteins C3, factor B and C2 by human monocyte-derived macrophages, using biosynthetic labelling, immunoprecipitation, sodium dodecyl sulphate-polyacrylamide gel (SDS-PAGE) and autoradiography. There was a consistent increase in C3 production and secretion with both IgA and IgG immune complexes. This increase appeared after a 24-hr incubation period of the macrophages in the presence of immune complexes. No change in the biosynthesis of factor B and C2 proteins was observed in these experiments. Concomitant with the enhanced C3 biosynthesis, the immune complexes caused an increase in macrophage tumour necrosis factor (TNF) production; 310 + 24 U/ml/5 x 10(5) cells and 430 + 51 U/ml/5 x 10(5) cells for IgA and IgG immune complexes, respectively, versus 12 + 8 U/ml/5 x 10(5) cells in the control cells. The presence of prednisolone (2 x 10(-5) M) or dexamethasone (1 x 10(-7) M) inhibited the immune complex-induced TNF production, but had no effect on C3-increased synthesis, suggesting that the effect of immune complexes was not mediated by endogenous TNF production. These findings may be relevant to the local inflammatory response in IgA immune complex-mediated diseases, including IgA nephropathy.