Fifteen human renal allograft recipients were studied serially to determine the effect of rejection on the complement system, as measured by alterations in serum β10/β1A-globulin (C'3). β1C/β1A-globulin levels were elevated during the early weeks after transplantation: 58 ± 18% above the mean at the time of rejection, followed by a decline to 41 ± 16% below the mean 22 ± 6 days after rejection began. Changes in β1O-globulin levels were not associated with infection or with surgery per se or during periods of clinical stability with no rejection; on the other hand, unstable levels were present during chronic low grade rejection.