PGA mutase was crystallized in two forms. The activity of the crystalline enzyme was directly assayed by the polarimetric measure-ment, The optimum pH, the equilibrium constant at 25°C, and the Michaelis constant for the coenzyme were determined. Among the compounds structurally related to the substrate, only phosphoryl enolpyruvate inhibited the enzyme. In relatively high concentrations various metallic ions showed inhibitory effects, but the SH-inhibitors had little effect. Moreover the chelating agents given in relatively high concentrations inhibited the enzyme. It is then discussed whether the mutase may be the metal enzyme or not. Fluoride inhibited the activity. However, this inhibition was prevented by the addition of EDTA or citrate, whose concentration was higher than that of fluoride. The mechanism of fluoride inhibition is discussed.