Compared with several other enveloped viruses, purified virions of frog virus 3 contained a relatively high activity of a protein kinase which catalyzed the phosphorylation of endogenous polypeptides or added substrate proteins. Virions also contained a phosphoprotein phosphatase activity which released phosphate covalently linked to proteins. It was possible to select reaction conditions where turnover of protein phosphoesters was minimal, as the phosphatase required Mn(2+) ions for activity whereas the protein kinase was active in the presence of Mg(2+) ions. Electrophoretic studies in polyacrylamide gels containing sodium dodecyl sulfate indicated that at least 10 of the virion polypeptides were phosphorylated in the in vitro protein kinase reaction. Characterization of these phosphoproteins demonstrated that the phosphate was incorporated predominantly in a phosphoester linkage with serine residues. The protein kinase was solubilized by disrupting purified virions with a nonionic detergent in a high-ionic-strength buffer and was separated from many of the virion substrate proteins by zonal centrifugation in glycerol gradients. The partially purified protein kinase would phosphorylate polypeptides of many different animal viruses, and maximal activity was not dependent on added cyclic nucleotides. These properties distinguished the virion protein kinase from a well characterized cyclic AMP-dependent protein kinase which phosphorylated viral proteins only to a small extent.