Mesenchymal stem cell proliferation and differentiation are regulated by growth factors and systemic hormones in a concerted manner. We have examined the effects of platelet-rich plasma (PRP) and its interaction with 1,25(OH)(2) vitamin D-3 [1,25(OH)(2)D-3] on the differentiation and proliferation of human mesenchymal stem cells (hMSCs). PRP stimulated cell proliferation in a manner inversely proportional to the dose. Exposure to PRP inhibited hMSC osteogenic differentiation. Co-treatment with PRP and 1,25(OH)(2)D-3, however, synergistically stimulated osteogenic differentiation. Serial co-treatment with PRP and 1,25(OH)(2)D-3 showed that this synergistic effect was due to the modulation of a 1,25(OH)(2)D-3-induced differentiation by PRP rather than vice versa.