1. The inhibition of the catalytic subunit of protein phosphatase-1 (PP-1c) by the regulatory subunit of cAMP-dependent protein kinase II (RII) was studied. 2. Phosphorylation or thiophosphorylation of RII increased its inhibitory potency up to 4- and 6-fold and rendered it competitive with respect to the substrate of PP-1c, phosphorylase a. The Ki values for thiophospho-RII and phospho-RII were 200 and 500 nM, respectively. 3. Though PP-1c was able to release phosphate from phospho-RII, its activity once incubated with phospho-RII, remained inhibited even 80% of the phosphate was released from phospho-RII. 4. The catalytic subunit of cAMP-dependent protein kinase was effective in suspending the inhibition employed either before or after the addition of phospho-RII to PP-1c. 5. No exclusive bindings of thiophospho-RII and heat-stable protein inhibitors to the PP-1c could be proved by double inhibition studies, however some synergism was observed in their effect.