Long-term cellular and humoral immune responses were studied in inbred rats in which bone was allografted. Transplants were made across major histocompatibility barriers. Mixed lymphocyte culture tests and humoral cytotoxicity assays were performed in order to determine degrees of cellular and humoral immunity. Grafts of complete bone and of bone free of marrow elicited both types of immune response and it is proposed that transplantation antigens for both types of reaction exist in the bone tissue itself. A gene-dose effect was found. Allogeneic bone grafts gave a stronger reaction than semiallogeneic grafts, and second-set grafts triggered a hypersensitivity type of humoral immune response. This study suggests that until the host-bone graft immunological relationship is better understood, the graft tissue should be matched to the host for major transplantation antigens to help prevent unexpected failures in massive bone allotransplantation.