Succinylacetone is a seven-carbon organic ketoacid that we have previously shown to inhibit tumor allograft rejection as well as the primary antibody response to sheep erythrocytes in rats. Because it appeared to be such a potent immunosuppressive agent in our initial studies, we evaluated succinylacetone for its ability to block graft-vs-host disease (GVHD) in adult F1 rats injected with parental strain spleen cells. Untreated ACE X Lewis F1 rats given Lewis strain spleen cells died of GVHD, with a mean survival of 24 days. By contrast, 62% of F1 rats in the group treated with succinylacetone survived, and the deaths that occurred in this group occurred significantly later. To confirm this observation and determine whether succinylacetone would interfere with bone marrow engraftment, lethally irradiated adult Wistar-Furth rats were reconstituted with syngeneic or totally allogeneic Fisher 344 lymphohemopoietic cells consisting of equal numbers of bone marrow and spleen cells. All animals given allogeneic cells without additional immunosuppressive treatment developed severe GVHD and died by day 42. By contrast 92% of the allogeneic cell recipients that had been treated with succinylacetone were long term survivors. Cytotoxicity typing of peripheral lymphocytes demonstrated greater than 90% donor-type lymphocytes persisting in the succinylacetone-treated recipients as long as 255 days post-transplantation. No chronic or late developing acute GVHD was observed even though the animals received succinylacetone as the sole immunosuppressant for only 28 days after transplantation. Furthermore, hemopoietic reconstitution in recipients of syngeneic cells was essentially identical between the control group and the group treated with succinylacetone. In addition, no gross or histologic evidence for renal, hepatic, cardiovascular, endocrine, or neurologic toxicity was observed in the long term transplant survivors treated with succinylacetone. These data indicate that succinylacetone treatment is effective in preventing lethal GVHD in this allogeneic bone marrow transplantation system while permitting normal hemopoietic reconstitution in the absence of significant chronic toxicity to other major organ systems.