The authors have elaborated a biochemical technique for the assessment of the selectivity of proteinuria by means of the clearances of ceruloplasmin, haptoglobin and albumin. The results obtained in 69 patients were compared to those established by the method of Cameron and Blanford, as well as by the clearance of alpha2-macroglobulin. No significant differences were found in the indices of proteinuria, as determined by the proposed biochemical technique and by the method of Cameron and Blanford. Thus the diagnostic value of both methods to assess the selectivity of proteinuria could be considered as equal. The additional measurement of the selectivity of proteinuria by means of proteins with considerably high molecular weight (haptoglobin, alpha2-macroglobulin) gave the possibility of recording cases with severely impaired glomerular permeability. In this respect, the use of the clearance of haptoglobulin, particularly the phenotype Hp 2--2, provided valuable diagnostic information in cases in which the routine methods gave borderline values for the index of proteinuria.