Incubation of phosphorylase kinase from rabbit skeletal muscle with cyclic AMP-dependent protein kinase and Mg-ATP causes a rapid phosphorylation of one serine residue on the P-subunit, followed by a phosphorylation of a further serine residue on the a-subunit [l-3]. The activation of phosphorylase kinase which accompanies this reaction is determined solely by the phosphorylation of the P-subunit [3-51. Nevertheless, the serine residue on the a-subunit, as well as that on the@ subunit, becomes phosphorylated in vivo in response to adrenaline [6], suggesting that it may have a physiological function. Several years ago we reported that extracts of rabbit skeletal muscle contained two different enzymes which dephosphorylated the P-subunit and a-subunit relatively specifically [7]. These enzymes were initially termed fl-phosphorylase kinase phosphatase and a-phosphorylase kinase phosphatase [7], but were subsequently renamed protein phosphatase-1 and protein phosphatase-2 [3]. Protein phosphatase-2 which had been purified several hundred fold was reported to dephosphorylate histones HI and H2b at similar rates to the a-subunit, and it also contained some glycogen synthase phosphatase and phosphorylase phosphatase activity [8]. Subsequently this enzyme was purified 2500-fold and shown to dephosphorylate a protein termed inhibitor-l [93, which is a potent inhibitor of protein phosphatase-1 [3].