There is evidence of two pathways of allorecognition, the direct pathway where T cells recognize intact allo-MHC molecules on the surface of target cells, and the indirect pathway where T cells recognize processed allo-MHC presented by self antigen-presenting cells. We used synthetic class II MHC allopeptides to study the cellular mechanisms of the indirect al-lorecognition pathway and its potential role in vascularized allograft rejection. LEW (RT11) rat T cells—which are primed by immunization with a mixture of four 25 mer class II MHC allopeptides, representing the full length sequence of the β chain of the hypervariable domain of the RT1.Du (WF), or by primary WF (RT1u) vascularized cardiac allografts—were capable of recognizing and proliferating to specific polymorphic class II MHC sequences when presented as peptides by responder APCs. T cells from naive LEW animals, WF animals immunized with syngeneic RTl.Duβ peptides, or LEW recipients of third-party BN (RTln) vascularized cardiac allografts did not proliferate to the RTl.Duβ peptides, indicating the specificity of al-lopeptide recognition. In the strain combination used, immunogenicity of class II MHC allopeptides is determined by factors other than polymorphisms alone, since epitopic differences in 2 of the 4 RT1.Duβ allopeptides were not immunogenic. Responder T cells were CD4+, and were inhibited by monomorphic anti-class II monoclonal antibodies, and by specific anti-class II alloantibodies. These observations confirm the occurrence of self MHC-restricted recognition of processed allo-MHC in primary vascularized allograft rejection, and provide the rationale to develop novel and specific immunotherapies in organ transplantation.