The sensitivity of the Coomassie Brilliant Blue (CBB) procedure differs for different proteins. To investigate the impact of this variability on clinical measurements of proteinuria, we collected urine specimens from patients with glomerular disease or drug-induced tubular nephrotoxicity and compared urinary protein concentrations as determined by the CBB method with those obtained by the biuret method. The CBB method underestimated the protein concentration predicted by the biuret method by as much as eightfold in specimens with low proportions of albumin. Determinations of protein by the CBB method in serially diluted urine specimens from patients with tubular damage also deviated from linearity by as much as twofold for protein concentrations less than 1 g/L. We conclude that CBB binding is relatively insensitive for measuring proteins specifically associated with tubular damage and hence may be inappropriate for clinical use.