We have reported previously that the glycogen-bound form of protein phosphatase-1 (termed PP-1G) is a heterodimer, composed of the 37-kDa catalytic (C) subunit complexed to a 103-kDa G-subunit that anchors the enzyme to glycogen [Stralfors, P., Hiraga, A. and Cohen, P. (1985) Eur. J. Biochem. 149, 295-303]. An antibody raised against a synthetic peptide corresponding to the phosphorylation site on the G-subunit was found to immunoprecipitate PP-1G specifically. No precipitation occurred if the antibody was preincubated with the synthetic peptide, or if PP-1G was replaced by either the isolated C-subunit or protein phosphatase-2A. The results confirm by a new and independent method that the G-subunit is complexed to the C-subunit, and that it is not a contaminant. The G-subunit is remarkably sensitive to proteolysis. At the final stage of purification, PP-1G was eluted as a broad peak of activity. The leading fractions contained the 37-kDa C-subunit and 103-kDa G-subunit, while the central and trailing fractions comprised the 37-kDa C-subunit plus a number of bands with molecular masses ranging over 40-80 kDa. The 40-80-kDa bands were phosphorylated by cyclic-AMP-dependent protein kinase and tryptic digestion generated the identical phosphopeptides obtained by trypsinisation of the 103-kDa G-subunit. Furthermore, antibody to the G-subunit immunoprecipitated protein phosphatase activity quantitatively in the leading, central and trailing fractions. The results demonstrate that the 40-80-kDa polypeptides are fragments of the G-subunit, and that fragments as small as 40 kDa retain the sites of phosphorylation as well as the ability to interact with the C-subunit and with glycogen. Khatra [J. Biol. Chem. (1986) 261, 8944-8952] reported that the glycogen-bound form of protein phosphatase-1 did not contain a G-subunit and that it was a dimer composed of two identical C-subunits. The present work has shown that this proposal is incorrect, and that proteolysis of the G-subunit to fragments that stain very poorly with Coomassie blue can explain why this error was made.