Both fresh and frozen allogeneic bone elicit both acellular and humoral immune response. This response includes the development of enhancing factors which block detectable immunity and probably protect the graft from rejection. There seems to be no evidence of an alteration in immunogenicity by freezing of the graft. The importance of these observations lies in the potential technique of employing fresh viable allografts; prior freezing and tissue matching for HL-A transplantation antigens should not be necessary.