The synthesis of C3, C2, factor B (B) C1-inhibitor and lysozyme has been studied in monocytes and macrophages isolated from the synovial fluids of patients with rheumatoid arthritis. Concentrations of all 5 proteins in culture supernatants were measured by the sandwich ELISA technique. Kinetic studies showed that only lysozyme and C3 could be detected in monocyte culture supernatants on the first day of culture, whereas C2, B and C1-inhibitor were not present until the third day. In contrast all 5 proteins could be detected in the supernatants of macrophage cultures on day 1. In both monocyte and macrophage cultures synthesis of lysozyme and C1-inhibitor continued throughout the culture period, whereas synthesis of C2, B and C3 appeared to be reduced after the fifth day in culture. Quantitative studies showed that the secretion rates of lysozyme (4,700 X 10(3) molecules/cell/hr) was similar in monocytes and macrophages. Synthesis rates for all 4 complement components in monocyte cultures were less than 0.2% of that for lysozyme. Although the synthetic rates were higher in macrophages, even then they constituted less than 2% of the rate for lysozyme. Synthetic rates for complement components, but not lysozyme, were increased by BSA-anti-BSA antigen-antibody complexes and reduced by serum-treated complexes. Although the functional activity of monocyte B was similar to that for serum, the activity of monocyte C2 was 5 times that of serum C2. As C42 formed with monocyte C2 had a half-life of 13.5 min at 30 degrees C, compared with 4.5 min for the enzyme formed with serum C2, it is probable that monocyte C2 is oxidized by the oxygen products of these cells.