α-Amylase contaminating partially purified potato phosphorylase was inhibited by M/5,ooo HgC1 2 without sensibly affecting phosphorylase activity. Phosphatase was simultaneously inhibited by M/5o NaF. The HgCl 2 prevented amylolysis of both added priming polysaccharide and synthesized amylose as shown by absence of maltose. At very low initial primer concentrations autocatalysis due to effective increase in primer concentration resulting from amylolysis was eliminated, but there was a rise in glucose production due to increased phosphatase action, and the ratio of glucose to phosphate produced was nearly I O. There was thus a marked interaction between phosphorylase and phosphatase activities, phosphatase being inhibited when high primer levels induced maximum initial phosphorylase activity. When both HgCl 2 and NaF were added the phosphorylase reaction, at high primer concentration, proceeded to equilibrium accompanied by only a very small increase in reducing power, and no glucose or maltose could be detected. At low primer concentration the reaction proceeded extremely slowly, and ceased when less than 10 per cent. of the glucose-1-phosphate had been converted. Equilibrium at about 75 per cent. conversion of glucose-1-phosphate was reached with primer concentrations from 10 to 500 mg. per cent., and the calculated mean chain length of the amylose produced was from approximately 1,000 units down to 20. At primer concentrations below 10 mg. per cent, the reaction virtually ceased when the calculated mean chain length was about 8oo units. The course of the reaction effected by potato phosphorylase when α-amylase and phosphatase were inhibited was thus analogous to that reported for crystalline muscle phosphorylase, except that the maximum chain length of the amylose was about 1,000 units as compared with 200 units for the muscle enzyme.