CD4 T cells play a crucial role in the acute rejection of MHC class II-disparate skin allografts, mainly by Fas/Fas ligand-mediated cytotoxicity. Because recent observations indicate that eosinophils may be found within allografts rejected by CD4 T cells, we evaluated the role played by IL-5, the main eosinophil growth factor, and by eosinophils in the rejection of MHC class II-disparate skin grafts. C57BL/6 mice rapidly rejected MHC class II-disparate bm12 skin grafts. Rejected skins contained a dense, aggressive eosinophil infiltrate. Lymphocytes isolated from lymph nodes draining rejected bm12 skin were primed for IL-5 secretion, and IL-5 mRNA was present within rejected grafts. The IL-5/eosinophil pathway played an effector role in allograft destruction, because the rejection of bm12 skin was significantly delayed in IL-5-deficient mice as compared with wild-type animals. The role of the IL-5/eosinophil pathway was further investigated in MHC class II-disparate donor-recipient strains unable to establish Fas/Fas ligand interactions. Fas ligand-deficient gld/gld mice rejected bm12 skins, and bm12 mice rejected Fas-deficient lpr/lpr C57BL/6 skins. Neutralization of IL-5 prevented acute rejection in both combinations. We conclude that MHC class II-disparate skin allografts trigger an IL-5-dependent infiltration of eosinophils that is sufficient to result in acute graft destruction.