ART-18, a mouse antirat IL-2R mAb inhibits IL-2 binding and IL-2-dependent T cell growth. Although both (LEW x BN)F1 kidney and heart allografts survive ca. 3 weeks in ART-18-treated LEW rats (acute rejection occurs within 10 days, P less than 0.001), the host responses against the two organs vary. In the heart model, the splenic CD4:CD8 ratio as determined by FMF was similar both in untreated and treated animals, but decreased significantly in kidney recipients conditioned with ART-18. In both mAb-modulated animal groups, splenocytes inhibited test MLR and prolonged test cardiac allograft survival in a donor-specific fashion upon adoptive transfer, suggesting that ART-18 mediates "sparing" of Ts. However, both CD4+ and CD8+ cells from kidney-grafted hosts conferred suppression in vivo; only the CD8+ subset was effective in the heart model. Immunohistologically, IL-2R+ cells were absent in the heart grafts of treated hosts; a significant proportion of the kidney cell infiltrate remained IL-2R+ despite continuous mAb administration. Although ART-18 therapy prolonged renal graft survival significantly, function was poor and the rats remained uremic. However, when one of the native kidneys was retained and the rat continued to enjoy normal renal function, IL-2R+ cells were abolished from the graft infiltrate, as shown by FMF and immunohistology. Thus, ART-18 treatment influences host responses differentially against kidney and heart allografts (modulation and depletion of IL-2R+ cells, respectively) despite increasing their survival comparably. The uremic state in the kidney model prevents elimination of infiltrating IL-2R+ mononuclear cells by a mAb directed specifically against them.