CD8(+) T lymphocytes are known to inhibit the development of eosinophilia and IL-5 synthesis in models of experimental lung disease. In transplantation, the rejection of fully mismatched cardiac allografts by recipients depleted of CD8(+) T cells is characterized by the recruitment of eosinophils in the rejected organs. We show here that this intragraft eosinophilia is dependent on the production of IL-5 since hearts transplanted into IL-5-deficient recipients depleted of CD8(+) cells did not contain eosinophils. More importantly, allograft survival was significantly extended in these animals. In mixed lymphocyte cultures (MLC), the presence of CD8(+) T cells in the responding cell population inhibited the secretion of IL-5. This inhibition was IFN-gamma dependent since adding neutralizing anti-IFN-gamma antibodies induced the production of IL-5. Furthermore, spleen cells isolated from IFN-gamma receptor (IFN-gammaR)-deficient mice secreted IL-5 upon allogeneic stimulation in primary MLC. In vivo, eosinophilia was observed in allografts rejected by IFN-gammaR-deficient recipients. On the contrary, grafts rejected by IFN-gammaR-deficient mice treated with neutralizing anti-IL-5 antibodies did not exhibit eosinophilic infiltration. Our study reveals the capacity of IL-5-secreting CD4(+) T cells and eosinophils to promote the rejection of heart allograft and demonstrates the importance of CD8(+) T cells and IFN-gamma in regulating this pathway of rejection.