Human monocytes and alveolar and peritoneal macrophages were cultured in serum-free medium with or without endotoxin (ET), agarose beads, or cycloheximide. The cell culture supernatants were collected after various intervals and examined by a monoclonal anti-S-protein antibody in Western blot and in a solid-phase enzyme immunoassay. We found that the phagocytes synthesize and secrete S-protein. ET stimulation or prolonged incubation of the cells did not favour S-protein production, which was inhibited by cycloheximide. Agarose stimulation increased the S-protein level in supernatants from monocyte but not from macrophage cultures.