The size of glycogen particles was decreased by treatment with trichloroacetic acid (TCA), phosphotungstic acid or hot 30% potassium hydroxide, but was not changed by treatment with 0.1 M or 0.5 M sodium hydroxide. Treatment with 2-mercaptoethanol did not disrupt large glycogen particles. Chymotrypsin, alkaline protease, and β-amylase did not affect the size of glycogen particles. Large glycogen particles were partially split by treatment with a-amylase. A large glycogen particle appears to be composed of several fundamental particles 20 -30 nm in diameter linked by α-1, 4-glucan chains. A model of the glycogen particle was proposed.