Disclosed is a method of inhibiting a rejection response by a primate to a transplanted organ. One exposes the primate to a mutant diphtheria toxin linked to anti-CD3 antibody so as to largely eliminate the host's peripheral blood T cell lymphocyte population. At the same time as, or after, the exposure step one administers to the primate's thymus gland donor lymphocytes. Transplantation of the organ follows. The primate is tolerized to the transplanted organ.