Publisher Summary Most tissues contain protein kinases that are stimulated several-fold by cyclic 3', 5'-adenosine monophosphate (cAMP) and catalyze the transfer of phosphate from ATP to several proteins. Incorporation of phosphate into protein can be monitored by transfer of 32p to protein from [γ -32 P] ATP in the presence of magnesium. The phosphorylated protein is separated from the labeled precursor by adsorption of the precipitated protein on filter paper disks and washing the disks as described by several investigators. Several proteins may be used as substrates in the assay, including muscle phosphorylase kinase and glycogen synthetase, protamine, adipose tissue lipase, casein, specific histones, and histone mixtures. When either phosphorylase kinase, glycogen synthetase, or lipase are used, the phosphorylation causes enzymatic activity changes which may be developed into alternate methods for assay of protein kinases. In practice, however, such assays are difficult to quantirate. A histone mixture is a suitable substrate for several reasons: (1) it is available from commercial sources, (2) there is little if any protein kinase contamination, (3) an adequate amount of phosphate is incorporated, (4) it is a stable and easily precipitable protein mixture, and (5) the degree of stimulation of histone phosphorylation by cAMP is usually relatively high.