One hundred kidney transplant recipients were evaluated on the first and fifth days after transplantation by Tc-99m mononuclear cell scintigraphy. We have developed a quantitative method to diagnose rejection and acute tubular necrosis (ATN) by comparing regions of interest drawn on allograft scintigraphs at different times after endovenous administration of the labeled cells. We suggest that the use of Tc-99m-WBC may be useful for the early diagnosis of rejection and the differential diagnosis of ATN.