Ultraviolet-B (UVB) irradiation of blood constituents intensifies their anti-rejection effect in pretransplant donorspecific transfusions. UVB-induced inhibition of the mixed lymphocyte reaction (MLR) between UVB-irradiated donor cells and prospective recipient cells is a predicator of this anti-rejection effect. In order to define the doseresponse relationship between the incident UVB irradiation on leukocyte concentrates and subsequent inhibition of their MLR responses, we collected 4 i 2 X lo9 leukocytes (93 2 7% lymphocytes) in 200 ml plasma from each of three volunteers by leukapheresis and exposed them to rapid, serial doses of UVB iuadiation which was delivered by a blood product irradiator (4R4440 UVB Irradiator, Baxter, Inc) with aliquots removed between doses. Lymphocytes from each aliquot were placed in MLR with panel donors and studied in three groups: 1) the panel donor cells were y-irradiated (1,500 rads) (i.e., only the UVB-irradiated cells could proliferate), 2) the UVBirradiated cells were y-irradiated (i,e., only the panel lymphocytes could proliferate), and 3) no y-irradiation (i.e., both cell populations could proliferate). Each group had a similar UVB dose-related diminution in the MLR (p = .79, ANOVA). A single dose of 6 J/cm* extinguished the MLR to baseline in all groups. This dose should theoretically prevent transfused cells from producing either graft-versus-host disease or allosensitization, and might heighten their tolerogenic effect. This dose will be employed in our study of donor-specific leukocyte transfusion in clinical renal transplantation.