Urinary clearances of bovine albumin derivatives rose with their increasing negative charge. A mathematical model has been developed to assess the relative contributions of glomerular ultrafiltration and tubular reabsorption to overall urinary protein clearance. This model indicates that the increased urinary clearance seen with negatively charged bovine albumin derivatives was due to a large reduction in the amount of protein absorbed by the proximal tubular epithelium, which swamped the effects of a small decrease in glomerular ultrafiltration. Derivatives of bovine albumin with increasing negative charge were also cleared with correspondingly greater rapidity from the blood stream.