Analysis of membranes from a variety of tissues has revealed a widespread distribution of a protein phosphorylation system dependent on the presence of both Ca2+ and "calcium-dependent regulator" (CDR). This protein phosphorylation system has been studied in some detail in nervous tissue. Neuronal membranes contain a protein phosphorylation system that requires Ca2+ and a soluble heat-stable protein [Schulman, H. & Greengard, P. (1978) Nature (London) 271, 478--479]. This protein has been purified to homogeneity from bovine cerebral cortex, with use of an assay based on its ability to stimulate Ca2+-dependent protein phosphorylation in membranes. This protein kinase activator appears to be identical to CDR of cyclic nucleotide phosphodiesterase. Throughout its purification, this single entity was found to activate both Ca2+-dependent protein kinase and cyclic nucleotide phosphodiesterase. The kinase activator purified here and authentic CDR were equally effective in their ability to activate Ca2+-dependent protein kinase.