The particulate glycogen–glycogen synthetase complex isolated from mammalian liver is often strongly dependent on the addition of primer for activity. The present experiments offer an explanation for this behavior. Hepatic α-amylase, which is also adsorbed on the particle, can, even at the low temperatures (0–4°) of the preparatory procedure, hydrolyze the outer branches of particulate glycogen and thus reduce its efficiency as a primer. Synthetase activity of particulate glycogen prepared from the livers of starved animals is much more dependent on the addition of exogenous primer as compared to the enzyme from fed animals. Correspondingly, the livers of starved animals contain a lower amount of particulate glycogen and exhibit higher α-amylase activity.When particulate glycogen is rapidly prepared from fed animals, the glucose-6-P independent portion of its glycogen synthetase activity is only slightly increased by addition of extra primer. The glucose-6-P dependent activity of the same preparations, however, is doubled by addition of soluble glycogen, suggesting that the latter form of the synthetase is bound to glycogen in a partially inactive complex.The efficiency of a glycogen sample in binding the synthetase seems to be proportional to the ability of that sample to act as primer in the synthetase reaction. This finding strengthens the hypothesis that the enzyme is adsorbed to glycogen through its catalytic site, i.e. in an enzyme–substrate complex. It is evident, however, that in a number of cases this complex is only partially active since it exhibits varying degrees of dependence on added soluble primer.