SUMMARYKidney's grafted from AS to Ag-B-incompatible AS2 rats are known to survive for many months without immunosuppressive treatment (11). The cell-mediated immune status of the spleens of such recipients was analyzed with the popliteal lymph node assay of graft-versus-host reaction. Likewise, for comparison, classically tolerant AS2 rats were analyzed. The kidney recipients showed normal or slightly reduced reactivity, but none behaved like the truly tolerant controls. The overall slight reduction of reactivity found in the kidney recipients is comparable to what has been found earlier (3) in the same strain combination during attempts to immunize with skin grafts followed by s.c. injections of lymphoid cells. The nature of this slight reduction in reactivity is unknown. Three long surviving heart grafts were performed between Ag-B-compatible strains and the recipient spleens were analyzed as in the other strains, at 9-13 months after operation. The results were again compatible with active immunization rather than with acquired tolerance. The long-term survival of organ grafts in spite of a vigorous cellular reactivity against the donors' antigens is perhaps explainable in terms of enhancing antibodies.