The first component of complement, C1, was produced by human as well as guinea pig macrophages. Supernatants from serum-free cultured macrophage monolayers, tested separately for C1q and C1 activity, showed a ratio of 15:1, respectively, and a different time course of C1q and C1 production, indicating that the subcomponents of C1 are synthesized independently. Heat-inactivated fetal calf or guinea pig serum (2 hr, 56 degrees C), added to the culture medium, were found to be inhibitory for C1 but not for C1q activity, obviously due to still active C1 inhibitor in these sera. De novo synthesis was confirmed by reversible inhibition of C1 and C1q production by cycloheximide (0.5 microgram/ml) and puromycin (1 microgram/ml), by incorporation of radiolabeled amino acids into the C1s subcomponent, and by uptake of incorporated radioactivity by EA, which was sensitive to EDTA. In additional experiments, 2,2'-dipyridyl markedly reduced C1q and also C1 secretion in the supernatants. Inhibition of the prolyl and lysyl hydroxylation and, as a consequence, structural instability of the collagenous region in the C1q molecule, is discussed as the cause of this effect which is analogous to collagen biosynthesis.