The present study was conducted to assess whether immune complex (IC) activity, measured by a complement consumption assay (CC) could be used to indicate a forthcoming rejection crisis in the early phase after renal transplantation. Complement factor B and C4 determinations by immunoassays were performed also. Consecutive specimens were drawn from each patient (n = 14) from zero to nineteen days after transplantation. In seven of the fourteen patients elevated levels of IC activity were demonstrated prior to transplantation. About three weeks later only 2 of the patients had demonstrable IC in circulation. Activated factor B (B) was found in 11 of the patients just before transplantation; however, only three of the patients were factor B positive nineteen days later. During the same observation time ten of the patients (71%) had indication of one or several rejection crises. With a few exceptions only minor individual fluctuation in C4 serum-concentrations were seen. It is concluded that circulating IC occurred in a rather high percentage of patients with glomerulonephritis or pyelonephritis and that IC determination by the CC assay had no value in the prediction or diagnosis of rejection episodes following renal allotransplantation.