Immunological enhancement—the prolonged survival of tumour or organ allografts brought about by passive immunization of the host with alloantibody directed against the graft's antigens—has not yet been applied successfully to clinical organ transplantation. There are many reasons for this, among them the difficulty of preparing the required battery of alloantisera and the danger of causing hyperacute rejection of kidney allografts. We show here that it should be possible to circumvent both these problems by using a xenogeneic antibody directed against a well defined hapten rather than against histocompatibility antigens.