Injection of parental strain rat lymphocytes under the kidney capsule of semi-allogeneic F1 recipients causes a local graft-versus-host reaction (GVHR) characterized by a heavy mononuclear cell infiltrate and renal tubular destruction. Since the cellular events involved may have relevance to allogeneic tissue damage in GVH disease and allograft rejection, a detailed analysis of the rat renal GVH reaction was performed. A purified CD4+ lymphocyte subpopulation was as effective in mediating a local GVHR as unfractionated parental lymphocytes, but neither naive CD8+ nor specifically sensitized CD8+ lymphocytes produced a detectable renal GVHR. Mononuclear cells harvested from renal GVHR lesions induced by CD4+ lymphocytes were able to lyse natural killer (NK)-sensitive targets when tested in vitro, but showed no allospecific cell-mediated cytotoxicity. Experiments using recombinant PVG rats demonstrated that the ability of the injected cells to cause a GVHR was dependent upon a disparity in MHC class II antigens and that an isolated disparity of MHC class I antigens alone was not a sufficient stimulus to provoke a response. The use of chimaeric rats demonstrated that F1 MHC alloantigens present on kidney parenchyma (but absent on bone marrow-derived cells) were not sufficient to stimulate injected parental lymphocytes, even in the presence of markedly increased amounts of MHC antigens on vascular endothelium and renal tubular cells following in vivo administration of interferon-gamma (IFN-gamma). These results suggest that the renal GVHR in the rat is mediated principally by the interaction of parental CD4+ lymphocytes recognizing and responding to class II F1 alloantigens on bone marrow-derived cells. The resulting tissue damage is most likely a result of a delayed-type hypersensitivity (DTH) phenomenon.