Abstract 1. 1. Both treatment with diethylpyrocarbonate and photo-oxidation with rose bengal produces the loss of the three enzymatic activities of rabbit muscle phosphoglycerate mutase: phosphoglycerate mutase, glycerate-2,3-P 2 synthase and glycerate-2,3-P 2 phosphatase. 2. 2. The synthase and the phosphatase activities are less affected than the mutase activity. 3. 3. Glycerate-2,3-P 2 and glycerate-3-P protect against diethylpyrocarbonate inactivation, but do not protect against inactivation produced by photo-oxidation. 4. 4. Hydroxylamine reactivates the diethylpyrocarbonate-treated enzyme. 5. 5. Chemical modification of phosphoglycerate mutase markedly reduces its ability to form the functionally active phosphoenzyme. 6. 6. These results provide evidence of the intrinsic character of the three enzymatic activities of phosphoglycerate mutase. In addition, they support the existence of two separate binding sites for monophosphoglycerates and for bisphosphoglycerates.