Varicella-zoster virus (VZV) glycoprotein gpl is the predominant viral glycoprotein within the plasma membranes of infected cells. This viral glycoprotein is phosphorylated on its polypeptide backbone during biosynthesis. In this report, we investigated the protein kinases which participate in the phosphorylation events. Under in vivo conditions, VZV gpl was phosphorylated on its serine and threonine residues by protein kinases present within lysates of either VZV-infected or uninfected cells. Because this activity was diminished by heparin, a known inhibitor of casein kinase II, isolated gpl was incubated with purified casein kinase II and shown to be phosphorylated in an in vitro assay containing [y-32P]ATP. The same glycoprotein was phosphorylated when [32P]GTP was substituted for [32P]ATP in the protein kinase assay. We also tested whether VZV gpl was phosphorylated by two other ubiquitous mammalian protein kinases-casein kinase I and cyclic AMP-dependent kinase-and found that only casein kinase I modified gpl. When the predicted 623-amino-acid sequence of gpl was examined, two phosphorylation sites known to be optimal for casein kinase II were observed. Immediately upstream from each of the casein kinase II sites was a potential casein kinase I phosphorylation site. In summary, this study showed that VZV gpl was phosphorylated by each of two mammalian protein kinases (casein kinase I and casein kinase II) and that potential serine-threonine phosphorylation sites for each of these two kinases were present in the viral glycoprotein.