Platelet-derived growth factor (PDGF), a peptide isolated from human platelets, was studied for its effects on bone formation by examining the synthesis of DNA, collagen and noncollagen protein in cultures of 21 day fetal rat calvaria. PDGF, at concentrations of 0.6-200 ng/ml, increased the incorporation of 3H-thymidine into acid insoluble residues in calvaria cultured for 24-96 hr; PDGF also increased the bone DNA content. After 24 hr of culture, PDGF caused a dose-dependent stimulation of 3H-proline incorporation into noncollagen protein (NCP) and a small stimulation of its incorporation into collagenase-digestible protein (CDP). After 96 hr of culture, PDGF did not affect NCP but inhibited the labeling of CDP. The PDGF stimulatory effect on the incorporation of 3H-thymidine was seen in both the periosteum and periosteum-free calvarium while that on the labeling of NCP was seen in the central nonperiosteal bone. Histological sections showed a threefold increase in the mitotic index after colcemid arrest in PDGF-treated bones. The stimulatory effect of PDGF on the incorporation of 3H-thymidine was enhanced by cortisol. These studies indicate that PDGF stimulates bone DNA synthesis and has a no specific stimulatory effect on protein synthesis in short-term cultures.