Acute allograft rejection is an adverse predictor for long-term graft survival in kidney transplantation. The goal of this study is to discover diagnostic biomarkers of acute allograft rejection in kidney transplant patients to identify those of whom have acute rejection (AR) using blood samples. An 13-peptide biomarker panel was identified that can discover acute kidney allograft rejection. The panel was validated in an international cohort of patients. Results hold great potential to transition from proteomic discovery to routine clinical use in order to monitor post-transplant patients.