The role played by antigenic peptides bound to major histocompatibility complex (MHC) molecules is evaluated with H2-DMalpha(-/)- mice. These mice have predominantly class II-associated invariant chain peptide (CLIP)-, not antigenic peptide-bound, MHC class II. H2-DMalpha(-/)- donor heart grafts survived three times longer than wild-type grafts and slightly longer than I-A(beta)(b)-(/)- grafts. Proliferative T cell response was absent, and cytolytic response was reduced against the H2-DMalpha(-/)- grafts in vivo. Residual cytolytic T cell and antibody responses against intact MHC class I lead to eventual rejection. Removal of both H2-DMalpha and beta2-microglobulin (beta2m) in cardiac grafts lead to greater (8-10 times) graft survival, whereas removal of beta2m alone did not have any effect. These results demonstrate the significance of peptide rather than just allogeneic MHC, in eliciting graft rejection.