Pepsinogen A (PGA) isozymogens are low molecular weight proteins that are present in serum and urine. The differences in the molecular structure of PGA-isozymogens involve only 2-4 amino acid substitutions. In a previous study, performed in 13 subjects only, a remarkable difference between the fractional renal clearances of the main PGA-isozymogens (PGA-3, PGA-4 and PGA-5) has been demonstrated. The aim of the present study was to further investigate these differences in fractional clearance between PGA-isozymogens and to determine whether these differences are caused by differences in glomerular sieving. For this purpose the fractional clearances of PGA-isozymogens were measured in 57 subjects. In accordance with the previous study, the median fractional clearance of PGA-5 (13%) was lower than the median fractional clearance of PGA-4 (17%; p < 0.02) and the median fractional clearance of PGA-4 was lower than the median fractional clearance of PGA-3 (26%; p < 0.001). The glomerular sieving coefficients of PGA-isozymogens were measured in 11 subjects during an elective heart catheterization by means of the fractional renal extraction method. No significant difference between the glomerular sieving coefficients of PGA-isozymogens could be demonstrated, being 0.81 for PGA-3, 0.96 for PGA-4 and 0.84 for PGA-5. It is concluded that the differences in renal handling between PGA-isozymogens must be explained by differences in tubular reabsorption. These differences in tubular reabsorption between PGA-isozymogens support the hypothesis that positively charged amino acid residues of proteins are involved in the tubular protein reabsorption.