SUMMARYProtein kinase has been extracted in soluble form from virions of pseudorabies virus using 10~ NP40, 0.6 M-NaC1. Chromatographic analysis of the extract on DEAEcellulose and on phosphocellulose showed it to contain more than one kinase. The activity responsible for the phosphorylation of the major phosphoproteins (mol. wts. 120000, 115000 and 72000) of virions was found to be similar in its properties to the host enzyme casein kinase I1. Purified casein kinase II from ascites cells or from pig liver was able to phosphorylate heat-inactivated virions. In addition to the major phosphoproteins, active virion preparations were able to phosphorylate a minor low molecular weight phosphoprotein, incorporation into which could be stimulated by the addition of cyclic AMP to the assay. Purified host cyclic AMP-dependent protein kinase also phosphorylated this protein in heat-inactivated virions. Analysis of herpes simplex virus type 1 showed that the major phosphoproteins (VP12 and VP23) could be phosphorylated in heat-inactivated virions by added casein kinase II. One of these (VP12) together with a further minor phosphoprotein (VP 14) could be phosphorylated by cyclic AMP-dependent protein kinase.