Gene UL13 of herpes simplex virus type 1 (HSV-1) has previously been proposed to encode a protein kinase. An HSV-1 mutant with UL13 inactivated by insertion of the Escherichia coli lacZ gene was constructed. This UL13-lacZ mutant was found to grow to near wild-type (wt) titres in tissue culture. Comparison of silver-stained SDS-PAGE profiles of wt and UL13-lacZ virions demonstrated that the UL13 protein is a readily detectable component of wt virions, located in the tegument and probably equivalent to the previously described species VP18.8. Studies of in vitro phosphorylation with nuclear extracts of virus-infected cells and with detergent-treated virions showed that the UL13 protein is involved in phosphorylation of the tegument protein VP22. Extracts of cells engineered to express UL13, and infected with UL13-lacZ virus, were also capable of VP22 phosphorylation.