The role of platelet-rich plasma (PRP) as a promoter of bone healing remains controversial. The hypothesis investigated was that PRP improves bone healing of a critical-size diaphyseal radius defect in a rabbit model. The bone defect was filled with a high-surface ceramic scaffold, calcium-deficient hydroxyapatite (CDHA), with the addition of allogenic PRP, mesenchymal stem cells (MSC) or both. PRP yielded better bone formation than the empty CDHA scaffold as determined by both histology and micro-computer tomography (p < 0.05) after 16 weeks, whereas no difference was observed on biomechanical testing. Similar behavior was found in samples with MSC; however, the combination of MSC and PRP did not further improve bone healing. Furthermore, the resorption of CDHA was improved by the addition of PRP, MSC and MSC/PRP, but there were no differences between the groups. The areas of bone formation were greater in areas adjacent to the bone resection areas and towards the intact ulna. In conclusion, PRP improves bone healing in a diaphyseal rabbit model on CDHA and the combination of CDHA. This study supports the allogenic use of PRP for bone healing as an off-the-shelf therapy.