Basic fibroblast growth factor (bFGF) has been shown to stimulate cell proliferation after vascular injury. The mitogenic activity of bFGF requires interactions with both a high affinity receptor and a cell-surface heparan sulfate proteoglycan. We tested the ability of platelet factor 4 (PF 4) and other platelet heparin-binding proteins to modulate bFGF-stimulated [3H]thymidine incorporation into fibroblasts. The supernatant of thrombin-stimulated platelets contained an inhibitor of bFGF-induced mitogenesis; this activity coeluted with PF 4 upon gel filtration, heparin-agarose, and ion-exchange chromatography. Purified thrombospondin and beta-thromboglobulin did not inhibit the mitogenic activity of bFGF. PF 4 inhibited the activity of 5 pM bFGF with 50% inhibitory concentration of 75 nM. Purified PF 4 also inhibited the basal incorporation of [3H]thymidine into 3T3 fibroblasts and the increased [3H]thymidine incorporation occurring after wounding of a cell monolayer. PF 4 did not affect the mitogenic activity of serum. Inhibition of bFGF activity by PF 4 could be overcome by exogenous heparin or chondroitin-4-sulfate, suggesting that inhibition of mitogenesis is caused by binding of PF 4 to cell-surface glycosaminoglycans. These results indicate that an important role of PF 4 released at sites of vascular injury and platelet activation is to control cellular proliferation caused by the release of bFGF from ruptured cells.