The use of 8-methoxypsoralen (8-MOP) and UV-A irradiation to inactivate contaminating donor leukocytes in platelet concentrates and to prevent primary alloimmunization against donor class I major histocompatibility (MHC) antigens in mice was investigated. CBA/CaH-T6J mice with the H2k haplotype and BALB/cByJ mice with the H2d haplotype were used as donors and recipients, respectively. The mixed leukocyte reaction between these two strains of mice showed that treatment of spleen cells with 500 ng/mL 8-MOP and 5J/cm2 UV-A inhibited 99% of responder and 92% of stimulator function. There was no measurable loss of platelet aggregating activity after the treatment. After two weekly transfusions of platelets without any treatment, 93% of control mice (n = 15) developed anti-H2k antibody. In contrast, only 33% of mice (n = 15) receiving platelets treated with 8-MOP and UV-A became alloimmunized. After six weekly platelet transfusions, all mice became alloimmunized. Nevertheless, the mean titers of anti-H2k antibody in sera of the treated groups were significantly lower than the control groups. One hour posttransfusion recoveries of 51Cr-labeled donor platelets were also higher in mice transfused with the treated platelets. Thus, the pretreatment of platelet concentrates with 8-MOP and UV-A irradiation effectively reduced the alloantigenicity of class I MHC molecules. The implication of this finding in relation to the mechanism by which donor leukocytes allosensitize recipients is discussed.