Protein Phosphatase inhibitor-I becomes inhibitory toward protein phosphatase-I only when it has been phosphorylated by cAMPdependent protein kinase, and is likely to play an important role in the control of glycogen metabolism by epinephrine in skeletal muscle. Inhibitor- 2 is a subunit of the cytosolic form of protein phosphatase-l, termed phosphatase-11. Inhibitor-1 and inhibitor-2 are assayed by their ability to inhibit the dephosphorylation of phosphorylase a catalyzed by the free catalytic subtunit of protein phosphatase-1. They are effective at nanomolar levels, which is similar to the concentration of protein phosphatase-1 in the assays. Consequently, the extent of inhibition decreases as the concentration of protein phosphatase-1 increases and vice versa. It is, therefore, important to standardize the protein phosphatase concentration in the assays. Inhibitor-1 is in its dephosphorylated form at each stage of the preparation and must, therefore, be phosphorylated prior to assay. Inhibitor-2 is also diluted in solution A + 0.03% (w/v) Brij-35 and assayed in an identical manner to inhibitor-l, except that prior phosphorylation with cAMP-dependent protein kinase is omitted, and preincubation with protein phosphatase-1 is for 10 rather than 2 min, before addition of 32p-labeled phosphorylase a.