A Ca2+, calmodulin-dependent protein kinase from brain with a Mr of 640 000 is capable of phosphorylating glycogen synthase from skeletal muscle. The reaction was inhibited by the addition of 1 mM EGTA and 50 microM trifluoperazine, but not by protein kinase inhibitor and heparin. The amount of phosphate incorporated into glycogen synthase was 1.4 mol/mol subunit. The phosphorylation sites of glycogen synthase were cyanogen bromide-treated peptides CB-1 and CB-2 and only the seryl residue was phosphorylated.