A low molecular weight beta(2)-globulin (beta(2)-microglobulin), albumin, and total protein were measured in concentrated 24-hr urine specimens from 20 healthy subjects and 30 patients with clinical proteinuria of glomerular or tubular type. Classification of proteinuria was made on the basis of clinical diagnosis and size distribution of urinary proteins after gel chromatography. The molecular radii (Stokes' radii) of beta(2)-microglobulin and albumin, estimated by gel chromatography, were 15 A and 35 A. The average 24-hr urinary excretion in healthy subjects was 0.12 mg for beta(2)-microglobulin, 10 mg for albumin, and 80 mg for total protein. The patients with renal glomerular disorders had normal or only somewhat increased excretion of beta(2)-microglobulin, despite considerably increased excretion of albumin and total protein. Most of the patients with tubular dysfunction excreted large amounts of beta(2)-microglobulin, although they excreted normal or only slightly increased amounts of albumin and only moderately increased quantities of total protein. Consequently, the ratio or urinary albumin/urinary beta(2)-microglobulin was high in glomerular proteinuria (1100: 14,200), intermediate in normal proteinuria (33: 163), and low in tubular proteinuria (1.0: 13.3). Determinations of urinary clearances of beta(2)-microglobulin and albumin in four healthy subjects and 11 patients indicated that increased excretions of the two proteins were associated with increased clearances. The results suggest that quantitative determinations of urinary beta(2)-microglobulin and urinary albumin may be useful for detecting disorders of the renal handling of plasma proteins. The findings also seem to suggest a selective tubular reabsorption of the two proteins. Estimates on sera revealed a close correlation between serum levels of beta(2)-microglobulin and creatinine and also a greatly raised serum concentration of beta(2)-microglobulin after bilateral nephrectomy.