Recent technologic advancement enables us to prepare leukocyte-depleted or UVB-irradiated platelet concentrates for possible prevention of primary HLA alloimmunization. However, it is yet not known which of these two approaches is more efficacious. Because well-controlled studies cannot be easily conducted in human subjects to answer this question, a series of experiments were performed using a mouse transfusion model. The results showed that 100% of CBA mice with H2k haplotype developed antibody to donor H2d major histocompatibility complex (MHC) antigens after two weekly transfusions of platelet concentrates containing 2000 leukocytes/microL. In contrast, only 50% of the mice became alloimmunized after receiving platelets containing < or = 2 leukocytes/microL. More impressively, none developed anti-H2d antibodies after receiving two platelet concentrates irradiated with 1,200 mJ/cm2 UVB. UVB irradiation was found to be equally effective in reducing the alloantigenicity of platelet concentrates regardless of whether they contained more than a fully immunogenic dose of leukocytes. The antibody titers determined after five weekly transfusions also supported the observation that UVB irradiation was more efficacious than a 3–log leukocyte depletion in the prevention of primary alloimmunization to MHC antigens. In addition, the studies showed that only transfusions of UVB-irradiated platelet products could induce the suppression of immunologic responses to donor MHC antigens in recipients and the induced immunologic suppression could not be further enhanced by gamma irradiation or by leukocyte depletion.