Proteinuria was studied in ten renal allograft recipients; it was defined as: (a) glomerular--characterized by predominant albumin excretion; (b) tubular--significant excretion of both albumin and low molecular weight (LMW) proteins; and (c) glomerulo-tubular or mixed type, a combination of the two. LMW protein and albumin were quantitated by polyacrylamide gel electrophoresis with sodium dodecyl sulfate. In the immediate posttransplant period, LMW protein and albumin excretion, expressed as a percentage of creatinine clearance, were high, revealing a mixed pattern, and excretion of both protein classes was higher than during both acute tubular necrosis and acute rejection crisis. Tubular proteinuria was observed in acute tubular necrosis; a glomerulo-tubular or mixed pattern of protein excretion in acute rejection crises.