Platelet-derived growth factor (PDGF) influences the proliferation and differentiation of a variety of cells. In this study, we have investigated the effect of PDGF isoforms on chondrogenesis by stage 24 chick limb bud mesoderm cells in culture. Synthesis of sulfated proteoglycans, an index of chondrogenesis, was inhibited by all three PDGF isoforms (PDGF-AA, PDGF-AB, and PDGF-BB). Application of PDGF isoforms during the first 2 days of culture, before the cells were overtly differentiating, resulted in decreased synthesis of sulfated proteoglycans. This was similar to when PDGF isoforms were present throughout the culture period. However, application of PDGF isoform during only the last 2 days of culture, did not inhibit cartilage matrix production. When chondrogenic and nonchondrogenic cells were separated from the cultures and replated, PDGF-AB and PDGF-BB inhibited the incorporation of sulfate by the chondrogenic cells. Recombinant bone morphogenetic protein 2B reversed the inhibitory effects of PDGF on sulfated proteoglycan synthesis and DNA synthesis. PDGF receptor binding analysis indicated that beta-receptors were predominant receptors present on the chondrogenic and nonchondrogenic cells of the stage 24 mesoderm. PDGF isoforms increased thymidine incorporation by 48 h in both high and low density cultures. However, at later periods, cell proliferation was inhibited by PDGF-AA and PDGF-AB but not by PDGF-BB. PDGF acted as a bifunctional modulator of mesodermal cell proliferation and thus may regulate chondrogenesis during limb differentiation and morphogenesis.