Glomerular permeability to horseradish peroxidase, a protein slightly smaller than rat albumin but similar in shape, was studied in Wistar-Furth rats by using a purified neutral isozyme (HRP; molecular radius, a(e) = 29.8 A) as well as an anionic succinyl-derivative (sHRP a(e) = 31.8 A) and a cationized enzyme (cHRP, a(e) = 30 A). The clearance rate of the proteins was determined over a 20-min period using the amounts of enzyme actually filtered (i.e., protein in the urine and protein reabsorbed by tubules). Fractional clearance of cationic HRP (0.338 +/- 0.019) exceeded that of neutral HRP (0.061 +/- 0.005) by a factor of 5.5 and that of anionic HRP (0.007 +/- 0.000) by a factor of 48. Tubular reabsorption was less than 10% of the filtered load. The experimental results indicate marked charge dependency of the filtration of proteins across the glomerulus. Fractional clearances for these proteins are significantly lower than those reported in the literature for dextrans of similar molecular radii. Other molecular properties such as shape and deformability may explain these differences.