Recent clinical and experimental evidence suggests that indirect allorecognition may promote the development of chronic rejection, but definitive experimental studies are lacking. To study the contribution of indirect allorecognition to chronic rejection, naïve Lewis (RT1(1)) rats were immunized with synthetic Wistar Furth (WF) class II-RT1(u).D (HLA-DR-like) or -RT1(u).B (HLA-DQ-like) or class I-RT1(u).A (HLA-A-like) peptides emulsified in complete Freund's adjuvant 7 d before transplantation (n = 5 to 7/group). Experimental and control animals then acted as recipients of fully mismatched WF vascularized cardiac allografts. Recipients received immunosuppression in the form of cyclosporine at a tapering dose that allows for long-term allograft survival. Animals were sacrificed at either 3 or 6 mo, with allograft arterial luminal occlusion scored on elastin stains by a blinded observer. At 3 mo, mean vessel scores were significantly higher in the RT1(u).A-immunized versus class II-immunized and control groups (P < 0.05). By 6 mo, there was progression of chronic allograft vasculopathy and a significantly higher mean vessel score in the RT1(u).A- and RT1(u).D-immunized versus RT1(u).B and control groups (P < 0.05). In vitro studies show evidence of shifting MHC allopeptide immunogenicity. It was concluded that T cells primed by specific donor class I and II MHC allopeptides promote the development of chronic vascularized allograft rejection. These novel observations provide definitive evidence of a link between indirect allorecognition and the development and progression of chronic rejection.