The experimental conditions in the potentiometric method for the determination of the iodine-binding capacity (Ib) of starch and amylose [R. L. Bates, D. French, and R. E. Rundle (1943) J. Amer. Chem. Soc. 65, 142-148] were not suitable for glycogen because of the much lower affinity for iodine of the latter. This difficulty was overcome by titration of small volume with both the iodine and glycogen at high concentration. Using the concentration cell circuit Pt electrode-blank-bridge-glycogen-Pt electrode, small increments of standard iodine solution were added to the blank solution and each was titrated to null by adding iodine to the glucogen solution [G. A. Gilbert and J. V. R. Marriott (1948) Trans. Faraday Soc. 44, 84-93]. Glycogen was determined by an anthrone-sulfuric acid method [F. W. Fales (1951) J. Biol. Chem. 193, 113-124]. Glycogens with Ib's ranging from 1.8 to 5.3% were observed.