Abstract A titrimetric method for the assay of glycogen phosphorylase is presented in which a direct and continuous course of reaction is obtained over a wide range of enzyme concentrations (7.2–378.3 μg/ml). The method resulted in rates which were in agreement with those obtained using the inorganic phosphate method, and the expected value of the equilibrium concentration ratio of inorganic phosphate to glucose-1-phosphate was obtained. The method can be extended to higher concentrations, and it can be used to measure the rate in either direction. The K m and V max values of each substrate, glucose-1-phosphate and inorganic phosphate, were determined.