An assay specific for the active form of liver phosphorylase kinase (EC 126.96.36.199) has been developed utilizing inhibition of the inactive form of phosphorylase kinase by β-glycerophos, phate and ethylene glycol bis(β-aminoethyl ether) N,N′-tetraacetic acid. Following in vitro activation the results compared favorably with those obtained using a less specific assay previously available. (J. R. Vandenheede, S. Keppens, and H. DeWulf, 1977, Biochim. Biophys. Acta. 481, 463-470; D. D. Doorneweerd, A. W. H. Tan, and F. Q. Nuttall, 1978, Diabetes 27, 474). The in vitro activation of phosphorylase kinase was not associated with the formation of a small-molecular-weight form of the enzyme. The utility of the assay in monitoring in vivo interconversion reactions in response to various physiological stimuli was demonstrated.