Mononuclear phagocytes synthesize chondroitin sulfate proteoglycan (CSPG), which is constitutively secreted. Because mononuclear phagocytes are known to interact with blood platelets, the effect of platelets on the release of CSPG in cultured human monocytes was investigated. After 6 days in vitro, the monocytes were supplied with fresh medium with different additions and subsequently exposed to [35S]sulfate for 24 hours before the medium fractions were harvested and analyzed for content of [35S]CSPG. Indirect evidence for the release of stimulatory factors from blood platelets was found when the addition of medium containing 50% serum made from platelet-rich plasma increased the expression of [35S]CSPG almost sevenfold compared with serum-free medium, whereas medium containing 50% serum made from platelet-depleted plasma increased the expression of [35S]CSPG about fourfold. Further, direct evidence for the stimulatory effect of platelets was found as the addition of autologous platelets to serum- free medium increased the expression of [35S]CSPG about threefold, and addition of supernatant from a corresponding number of thrombin- stimulated platelets was almost as efficient. The effect of five different platelet-derived factors (which are all present in serum) was investigated. Both platelet-derived growth factor (PDGF), platelet factor 4 (PF 4), and prostaglandin E2 (PGE2) used in physiologic concentrations were found to stimulate the expression of [35S]CSPG twofold to threefold, whereas transforming growth factor-beta had a slight inhibitory effect. 12-Hydroxyeicosatetraenoic acid had no significant effect on the expression of [35S]CSPG. Further evidence for the stimulatory effect of PDGF, PF 4, and PGE2 was found as serum depleted of these factors had significantly less stimulatory effect than control serum. The increased incorporation of [35S]sulfate into [35S]CSPG in cultures stimulated with serum or platelet-derived factors was not due to differences in molecular size or extent of sulfation of the proteoglycan molecules.