A monoclonal antibody recognizing Lyl, the murine homologue of CD5, was labeled with 90Y. In vivo biodistribution studies showed that 90Y-anti-Lyl selectively localized in lymphoid tissue. Groups of B10.BR mice (H-2k) were lethally irradiated and given major histocompatibility complex-disparate C57BL/6 (H-2b) bone marrow and spleen cells to induce graft-versus-host disease (GVHD). Eight days later, mice with active GVHD were administered a single i.p. injection of 50 μCi90Y-anti-Lyl. Fifty % of these mice were alive 2 months after treatment. Long term (≻4-month) survival was significantly higher than in phosphate-buffered saline-treated mice. Survival was slightly improved in groups of mice receiving control irrelevant antibody labeled with 90Y or mice receiving free 90Y. However, survival in these groups was not significantly different from the phosphate-buffered saline-treated control group. The improved survival was supported by data showing improved mean animal weight. An anti-GVHD effect was confirmed by histopathological analysis. Unlabeled anti-Lyl monoclonal antibody at comparable doses to 90Y-anti-Lyl was not effective. Animals that died following 50-μCi treatment did not die of radiation toxicity, since all mice receiving 50 μi 90Y-anti-Lyl plus syngeneic bone marrow survived. The window of therapy was narrow in our studies, since 100 μCi 90Y-anti-Lyl did not confer any survival advantage. Animals that did survive long term were studied for evidence of alloengraftment and found to have high levels of circulating donor mononuclear cells. 90Y-Anti-Lyl localized in the spleen, thymus, liver, kidney, and bone marrow but not in the bowel, lung, muscle, or skin. Animals given similar doses of free 90Y, 90Y-anti-Lyl, or labeled irrelevant antibody eliminated free 90Y fastest, followed by 90Y-anti-Lyl and then labeled irrelevant antibody. Hematological analysis of peripheral blood from 90Y-anti-Lyl-treated mice showed reduction in total WBC counts, absolute lymphocyte numbers, and absolute neutrophil numbers on day 24 after treatment. Myelosuppression recovered by day 38. These findings indicate that Lyl-positive cells are involved in the effector phase of GVHD and that radiolabeled antibodies may be useful as cell-specific probes for studying the GVHD network. 90Y-Anti-Lyl protected recipients long term from lethal GVHD, and the fact that it had a rather remarkable inhibitory and selective effect on the lymphoid system of mice suggests that these agents may have broader application in the field of transplantation. © 1991, American Association for Cancer Research. All rights reserved.