Glycogen synthase b was purified from rabbit liver by a procedure involving isolation of the glycogen-enzyme complex, DEAE-cellulose chromatography, and affinity chromatography. The purified enzyme had a specific activity of 25 mumol of glucose transferred from UDPglucose into glycogen per min per mg of protein at 30 degrees C in the presence of 10 mM glucose 6-P, and appeared to be homogeneous by the criterion of polyacrylamide disc gel electrophoresis. The b form was convertible into the a form by a rabbit-liver protein phosphatase. A subunit size of 85 000 was determined by electrophoresis in sodium dodecyl sulfate and molecular weights of 183 000 +/- 20 000 and 170 000 +/- 21 000 were determined for the a and b forms of the enzyme, respectively. On conversion of the a into the b form, 1.13 mol of phosphate was incorporated per 85 000 g of protein. The degree of phosphorylation and loss of glycogen synthase a activity paralleled each other.