We investigated in a murine model whether UVB irradiation of lymphohemopoietic cells would prevent the development of graft-versus- host disease (GVHD). Preliminary experiments showed that spleen colony (CFU-S) formation by hemopoietic cells was preserved at UVB doses that eliminated lymphocyte proliferation. In a parent into F1 model, UVB irradiation (5 to 15 mJ/cm2) of spleen cells added to normal marrow cells prevented the development of GVHD, whereas all recipients given untreated spleen cells developed GVHD. Syngeneic recipients of marrow exposed to 2.5 to 10 mJ/cm2 of UVB achieved normal hemopoietic reconstitution. Based on these observations, B6D2 F1 (H-2b x H-2d) recipients were given 1,000 cGy of total body irradiation (TBI) followed by transplantation of 5 x 10(6) parental B6 (H-2b) bone marrow cells and 10 x 10(6) B6 spleen cells, either unirradiated or exposed to UVB before infusion. All mice transplanted with cells exposed to 10 or 12.5 mJ/cm2 of UVB survived without GVHD. At 2.5 and 5.0 mJ/cm2, mice showed signs of GVHD, beginning at day 30, and 100% and 80%, respectively, eventually developed chronic GVHD. At 7.5 mJ/cm2, mice had weight loss, from which 60% recovered and survived without GVHD, while 40% died with GVHD. At 15 mJ/cm2, some recipients died from graft failure, while some survived without GVHD. All surviving mice were complete donor-type chimeras. Spleen size and cellularity and in vitro lymphocyte responses correlated inversely with the development of GVHD. Mice without GVHD showed specific tolerance to skin grafts from the second parent strain, while animals with GVHD rejected their skin grafts. Thus, in a murine model UVB irradiation of transplanted hemopoietic stem cells allows for hemopoietic reconstitution and prevents GVHD.