The phenotypes of the haptoglobin (Hp), ceruloplasmin (Cp), group-specific component (Gc), transferrin (Tf), and third component of complement (C3) were determined simultaneously in the serum and urine of patients with proteinuria secondary to nephrotic syndrome of various types. In a large number of cases the patterns of Hp, Cp, and C3 phenotypes in the urine showed marked deviations from those in the corresponding serum either in the mobility or the number of their electrophoretic bands. The monomeric Hp and the Cp were found to have a very augmented urine/serum ration in some cases. Such differences were not detected in the electrophoretic appearance of the Gc and Tf phenotypes. Our results imply that in the proteinuria of the nephrotic syndrome, factors other than molecular weight interfere in the passage of proteins through the glomerular wall.