The method of discontinuous micro-disc-electrophoresis was used to characterize the proteinuria after kidney transplantation in man for the early diagnosis of acute rejection crisis. By daily analysis of proteinuria the increase of albuminuria to more than 1 g per 24 h was found to be a sure sign of a beginning rejection crisis. In one third of our cases the rejection crisis could be recognized 7 days and in 80% 3 days before clinical onset. Besides increased albuminuria the appearance of low molecular weight proteins in urine and in 50% gamma globulinuria are further criteria for the rejection crisis. The described method is also very useful for the differential diagnosis of rejection crisis and inflammation in addition to cytological examination of the urine.