Controversy exists as to the type of cells present in the urine during renal allograft rejection. In order to resolve this controversy as well as to evaluate the value of urine sediment examination as a means of detecting AR, we quantitated the different cells present in urine during AR using an immunoperoxidase technique and monoclonal antibodies reactive with lymphocytes, monocytes, granulocytes, glomerular epithelial, tubular, and urothelial cells. Urine sediment (n = 176) was examined serially over 3 months in 15 transplant recipients. There were 12 episodes of early posttransplant acute tubular necrosis and 21 episodes of AR. It was possible to detect AR as well as to distinguish AR from ATN. Lymphocyte and tubular cell excretions were increased significantly during AR. Excretion of urothelial cells was also significantly increased during most episodes of AR suggesting that rejection of ureters occurs concomitantly with rejection of the kidneys.