Phosphoglyceromutase (2,3-diphospho-D-glycerate: 2-phospho-D-glycerate phosphotransferase, EC 2.7.5.3) has been purified from both frozen and fresh chicken breast muscle. During purification it was found that substrate, 3-phospho-D-glycerate stabilized the enzyme against heat inactivation to almost the same extent as did the cofactor 2,3-diphospho-D-glycerate.Phosphoglyceromutase prepared from frozen chicken breast muscle separated into three peaks of activity (I, II, and III) following chromatography on DEAE-Sephadex in 0.05 μ phosphate buffer, pH 8.0, using a 0.0–0.4 M NaCl gradient. Each peak of activity was shown by polyacrylamide disc gel electrophoresis at pH 9.3 to contain two enzymically active components (isoenzymes Ia Ib, IIa IIb, and IIIa IIIb). Isoenzymes in the same peak had the same specific activity. Phosphoglyceromutase prepared from fresh chicken breast muscle yielded only one peak of activity following chromatography on DEAE-Sephadex. This peak contained two enzymically active components corresponding to isoenzymes Ia and Ib. Additional peaks of activity were not produced when phosphoglyceromutase from fresh muscle was subjected to freezing and thawing.Isoenzyme Ia and mixtures of Ia and Ib, IIa and IIb, and IIIa and IIIb were homogeneous in the ultra-centrifuge sedimenting as single peaks. The sedimentation coefficient obtained for isoenzyme Ia and for Ia and Ib combined was 4.15 S, the diffusion constant 6.62 × 10−7 cm2/s, and the molecular weight calculated from both gel filtration and sedimentation data was of the order of 59 000. These results were confirmed by charge isomer studies which also showed that the isoenzymes of phosphoglyceromutase from frozen chicken breast muscle were proteins of the same size but different net charges.