The aim of the present study was to evaluate the influence of a pretreatment of the heart and kidney donor with the photosensitizer 8-methoxypsoralen plus ex vivo longwave ultraviolet irradiation of the graft (PUVA) on survival time and immunogenicity of rat heart and kidney allografts. PUVA pretreatment significantly prolonged the survival of these transplants in allogenic recipients. Furthermore, a synergistic effect with conventional immunosuppression (azathioprine, cyclosporine) was demonstrated. Immunohistological studies using monoclonal antibodies showed a significant reduction of MHC class II antigen expression in heart cryostat sections after PUVA treatment. In clinical kidney transplantation the number of rejection episodes was significantly lower in the first 3 months in the PUVA-treated patient group.