Rabbit skeletal muscle contains two phosphorylase kinase isozymes arising from the two different muscle types, the white and the red muscle (Jennissen, H. P., and Heilmeyer, L. M. G. (1974) FEBS Lett. 42, 77-80). The two phosphorylase kinase isozymes could be separated by affinity chromatography on a calmodulin-Sepharose 4B column. In media containing high concentrations of Ca2+, about 2 mM, both isozymes were bound to the affinity column. When the column was eluted with a buffer containing 0.2 mM Ca2+, the red muscle isozyme was eluted, whereas white muscle isozyme was eluted from the column by an ethylene glycol bis(beta-aminoethyl ether) N,N,N',N'-tetraacetic acid. The purified white muscle isozyme can be distinguished from the red muscle isozyme by its ability to inhibit calmodulin-dependent cyclic nucleotide phosphodiesterase. The two isozymes are also regulated differently by calmodulin. Both isozymes contain tightly bound calmodulin as a subunit (Cohen, P., Burchell, A., Foulkes, J. G., Cohen, P. T. W., Vanaman, T. C., and Nairn, A. C. (1978) FEBS Lett. 92, 287-293), which renders the enzyme sensitive to Ca2+. Only the white muscle isozyme can be activated by exogenous calmodulin.