This study investigates the ability of platelet-rich plasma (PRP) combined with biomaterials to enhance in vivo bone-repairing activity. A biodegradable hydrogel was prepared from gelatin, which has an affinity for various growth factors. Rabbit PRP was conventionally prepared by blood centrifugation and dropped onto freeze-dried gelatin hydrogel to obtain gelatin hydrogel incorporating PRP. Gelatin hydrogel incorporating PRP was applied to a bone defect of rabbit ulna to evaluate bone formation at the defect in terms of soft X-ray and histological examinations. As controls, fibrin incorporating PRP, empty gelatin hydrogel, and free PRP were applied to the defect; in addition, defect without any application was examined. Successful bone regeneration was observed at bone defect treated with gelatin hydrogel incorporating PRP, in marked contrast to the control groups. When in contact with gelatin, growth factors, such as platelet-derived growth factor and transforming growth factor beta(1), were released from the PRP. PRP growth factors are immobilized in the hydrogel through physicochemical interaction with gelatin molecules. The immobilized growth factors are released from the hydrogel in concert with hydrogel degradation. It is likely that the gelatin hydrogel permitted the controlled release of bioactive growth factors, resulting in factor-induced promotion of bone regeneration.