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 [beta] 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. Du [beta] peptides, or LEW recipients of third-party BN (RTln) vascularized cardiac allografts did not proliferate to the RTl. Du [beta] 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. Du [beta] 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.