The role of histocompatibility matching in bone allografting was studied in two canine bone graft models. In a cancellous ulnar segmental replacement model, frozen bone allografts exchanged between closely matched dogs were significantly better incorporated by radiographic and histologic criteria than were strongly incompatible grafts. Frozen allografts from disparate donors in recipients receiving immunosuppression appeared indistinguishable 6 months later from those in the untreated closely matched groups and from fresh autografts. Fresh vascularized orthotopically placed fibular bone grafts were evaluated by quantitative blood flow assessment, microangiography, and fluorochrome histomorphometry. Revascularized grafts exchanged between untreated closely matched dogs demonstrated preservation of blood flow and a pattern of repair that was delayed but not otherwise different than vascularized autografts. These results suggest that fresh vascularized grafts in the judiciously matched or immunosuppressed recipient offer attractive clinical possibilities.