BACKGROUND The biological actions of platelet-rich plasma (PRP) are thought to be mediated primarily by constituent transforming-growth factor-beta1 (TGF-beta1) and platelet-derived growth factor-AB (PDGF-AB). However, we previously demonstrated that type I collagen expression in periodontal ligament (PDL) cells is acutely stimulated through fibrin clot formation produced by the fibrinogen within PRP, rather than by the known growth factors. To investigate the possible involvement of other unidentified components in PRP action, we have now compared the effects of PRP with those of known recombinant growth factors on cell proliferation, alkaline phosphatase (ALP) activity, and collagen synthesis in human PDL cell cultures. METHODS PRP was prepared by an established two-step centrifugation protocol using blood obtained from adult human volunteers. Cells cultured in serum-reduced medium on native or collagen-coated plates were treated with PRP, TGF-beta1, or PDGF-AB. Cellular DNA synthesis was evaluated by bromodeoxyuridine incorporation. ALP activity was assessed using p-nitrophenylphosphate with formalin-fixed cells, and cellular DNA content was subsequently quantified using bis-benzimide. Collagen synthesis was evaluated using a specific dye-based assay kit. RESULTS 1) As did both TGF-beta1 and PDGF-AB, PRP stimulated cell proliferation. 2) However, only the initial mitogenic action of PRP was attenuated in collagen-coated plates. 3) PRP, but neither growth factor, immediately induced fibrin clot formation and subsequently stimulated cellular adhesion and collagen synthesis. 4) These effects were significantly augmented on collagen-coated plates. 5) PRP enhanced ALP activity, but neither TGF-beta1 nor PDGF-AB replicated this effect. CONCLUSIONS When evaluated versus the concentrations of growth factors known to be contained by our PRP preparations, these data support the concept that PRP-constituent TGF-beta1 acts as a significant growth factor on PDL cells. However, our findings also strongly suggest that the PRP-induced increase in ALP activity is mediated by an as-yet-unidentified component(s). In conjunction with previously demonstrated fibrinogen-mediated actions, our data provide evidence that PRP produces a number of potent effects on PDL cells that does not solely reflect simple combination of its major known growth factors.