To determine whether immune tolerance to donor major histocompatibility complex (MHC) antigens can be induced by transfusions of UVB-irradiated leukocytes, studies were conducted in inbred strains of mice with well-characterized MHC antigens. CBA mice with H-2k phenotype and BALB/c mice with H-2d phenotype were used as recipients and donors, respectively. Humoral immune tolerance is defined as absence of antibody response after challenge with transfusions of untreated donor leukocytes. It was found that transfusions of purified peripheral blood mononuclear leukocytes irradiated with 1,200 mJ/cm2 UVB not only prevented allo-immunization but also induced humoral immune tolerance to donor class-I and -II MHC antigens in all recipient mice. Donor plasma and platelets interfered with the induction of this tolerance. The tolerance induction by UVB-irradiated leukocytes is dose-dependent. Four weekly transfusions of 2 x 10(5) leukocytes were required to ensure tolerance induction in all mice. The results of a long-term follow-up study showed that the induced tolerance is long-lasting and can withstand repeated challenges by untreated donor leukocytes. Recipient mice tolerant to H-2d antigens also became tolerant to H-2b, H-2r, and H-2n MHC antigens but did not have impaired antibody responses to antigens unrelated to donor leukocytes. Adoptive transfer experiments showed the development of negative regulatory cells in spleens of the tolerized mice. In view of recent feasibility of using UVB-irradiated human platelet concentrates for prevention of HLA alloimmunization, the findings of this study support that UVB- irradiated donor leukocytes could have potential, important clinical application in transplantation medicine.