The present study was initiated to study the efficacy of donor pretreatment with interferon-γ to induce class II antigen expression in heart tissue, and to investigate whether this pretreatment would influence heart allograft survival. BN rats were used as donors, and LEW rats as recipients. During a period of 3 consecutive days prior to transplantation, IFN-γ was administered to BN rats via continuous intravenous infusion at dosages of 10, 10, and 5.10 U/hr. Control animals were infused with PBS; each group consisted of 9 animals. Analysis of IFN-7induced class II expression by immunoperoxidase staining revealed a significant, fourfold increase in the number of dendriticlike cells, irrespective of the IFN-γ dose given (controls: 15±4 vs. highest dose group: 57±9cells/mm;P< 0.005). Endothelial cells of arteries and venules remained class II antigen negative. Grafting of hearts from IFN-γ perfused donors to untreated recipients (6 animals per group), did not result in a shortened or prolonged survival time in any of the experimental groups, as compared to controls. These results indicate that upgrading of class II antigen expression on dendriticlike cells is not likely to be of importance for the process of rejection.