Purified glycogen synthase is contaminated with traces of two protein kinases that can phosphorylate the enzyme. One is protein kinase dependent on adenosine 3′: 5′-monophosphate (cyclic AMP) and the second is an activity termed glycogen synthase kinase-2 [Nimmo, H. G. and Cohen, P. (1974)]. Glycogen synthase kinase-2 has been found to be localized relatively specifically in the proteinglycogen complex. It has been purified 4000-fold by two procedures, both of which involve disruption of the complex, followed by DEAE-cellulose and phosphocellulose chromatographies. However the salt concentration at which glycogen synthase kinase-2 is eluted from DEAE-cellulose depends on the method that is used to disrupt the complex. The results indicate that glycogen synthase kinase-2 is firmly attached to a protein component of the complex. The isolation procedures separate glycogen synthase kinase-2 from phosphorylase kinase, cyclic-AMP-dependent protein kinase and other glycogen-metabolising enzymes. Glycogen synthase kinase-2 is the major phosvitin kinase in skeletal muscle, although glycogen synthase is a six to eight-fold better substrate than phosvitin under the standard assay conditions. Phosphorylase kinase and phosphorylase b are not substrates for glycogen synthase kinase-2.