This study assessed the capacity of a cartridge containing coated granular carbon to clear protein-bound solutes. Clearances for test solutes were measured while an albumin solution representing plasma was pumped from a 10 L reservoir through the cartridge at a rate of 200 mL/min for 5 h. Clearance values for phenol red, phenytoin, and indican were well below the limit imposed by the plasma flow and declined with time. The clearance of phenol red, which was the most tightly bound solute, fell from 38 +/- 12 to 17 +/- 2 mL/min. Additional studies revealed that the cartridge contained enough carbon to absorb all the protein-bound test solutes, but that the rate of their clearance was limited by the inability of granular carbon to take up solutes rapidly at a low concentration. The rate of solute uptake at low concentration was shown to be much greater when carbon was in powdered rather than granular form. A device in which approximately 50 g of powdered carbon was recirculated in the dialysate compartment of hollow fiber kidneys cleared phenol red and phenytoin more rapidly than the hemoperfusion cartridge containing 300 g of coated granular carbon. These results indicate that hemoperfusion over coated granular carbon provides limited clearance of protein-bound solutes.