Most filtered proteins are reabsorbed by the renal proximal tubule by a mechanism that involves binding to the brush border membrane and endocytosis. Under normal conditions the low-molecular-weight protein β<sub>2</sub>-microglobulin (β<sub>2</sub>M), which is used to detect tubular injury, is reabsorbed almost completely. However, in proteinuric patients an increased urinary excretion of β<sub>2</sub>M may not simply reflect tubular damage but might also result from a decreased tubular reabsorption due to competitive mechanisms. To examine the magnitude of such an effect we have studied the renal effects of albumin infusion (40 g in 2 h of a 20% solution) in 10 patients with a glomerular disease and proteinuria >3.5 g/24 h. Before, during and after albumin infusion the GFR (inulin clearance), RPF (PAH clearance), blood pressure and the urinary excretion of albumin, IgG, transferrin and β<sub>2</sub>M were measured. Albumin infusion resulted in a slight decrease of the GFR (72 ± 11 ml/min before and 67 ± 10 ml/min after infusion), an increase of the RPF (379 ± 66 ml/min before and 445 ± 83 ml/min after), a decrease of the filtration fraction (0.20 before and 0.17 after), and hemodilution. After infusion the urinary excretion of albumin increased from 4.5 ± 0.7 to 8.4 ± 1.6 mg/min (p < 0.05). The urinary excretion of IgG and transferrin increased, probably reflecting a change in glomerular size-selectivity. In contrast, the urinary excretion of β<sub>2</sub>M did not change significantly (baseline 12 ± 5 µg/min, end 13 ± 6 µg/min, percentage change 16.8 ± 11%). To correct for changes in tubular load we calculated the fractional reabsorption of β<sub>2</sub>M. The initial rise in albuminuria during infusion did not affect fractional tubular reabsorption (Δ%: 0.72 ± 0.52%, median 0.005%). In the period after infusion a slight decrease was noted (median –0.33%, p < 0.01). A decrease in the fractional reabsorption was particularly observed in patients with pre-existing tubular damage. In conclusion: infusion of albumin in proteinuric patients has no clinically relevant effect on the tubular reabsorption of β<sub>2</sub>M. Therefore, β<sub>2</sub>M is useful as a parameter to detect tubular injury and alterations in tubular handling of proteins in patients with proteinuria and glomerular diseases.