The differentiation of two types of renal graft rejection may be made early in incipient stages by means of a comprehensive renal function study. In the acute cell-mediated form, prolongation of orthoiodohippurate renal transit time as reflected in a diminished excretory index (El) is first observed, followed by a diminution in the effective renal plasma flow (ERPF). On reversal of the process, El precedes ERPF in returning to normal. In chronic humoral rejection, the principle feature is simple reduction of ERPF; transit time within the kidney appears normal. No reversal processes have been observed, although the process may be decelerated. At approximately 100 ml/min ERPF, these kidneys may undergo an acceleration of the clinical failure with rapid deterioration in El and in ERPF. At this point, surgical removal of the kidney has been carried out. © 1977, American Medical Association. All rights reserved.