Bone marrow mononuclear cell (BMMC) transplant may serve to produce donor specific tolerance for a coincident solid organ graft, but with the risk of graft versus host disease (GVHD). We examined in vitro the immunomodulatory effect of UVB on human BMMCs as potential prophylaxis against GVHD for clinical transplantation. After 10-200 J/m2 UVB-irradiation, BMMCs were examined by proliferative response (in mixed lymphocyte reaction and following phytohemagglutinin stimulation) and by cytokine profile. We also evaluated CFU-GM, CFU-GEMM, and BFU-E progenitor viability by 2-week methyl cellulose cultures following UVB-irradiation. Parallel studies were applied to marrow that was T-cell depleted by soybean agglutination (SBA) or by SBA and sheep erythrocyte rosetting (SBA-E-). We found that (1) UVB produces a dose-dependent inhibition of the proliferative response to stimulators by human BMMCs; (2) increasing doses of UVB-irradiation and increasing levels of T-cell depletion (TCD) are both inversely related to production of lymphokines (IL2, IL3, LIF, IFN-gamma, and GMCSF) and (3) T-cell depletion, but not UVB-irradiation, decreases the production of monokines (IL1, TNF, IL6). Progenitor cell viability was decreased but preserved at 100 J/m2 of UVB. Our findings suggest that UVB compares favorably with TCD as a technique for inhibition of GVHD and therefore that UVB-modulation of bone marrow (BM) inoculum may be useful in the prevention of GVHD in clinical bone marrow transplantation accompanying a solid organ graft.