Herpes simplex virus type 1 induces deoxycytidine deaminase (cytidine/deoxycytidine aminohydrolase, EC 3.5.4.5) activity when it lytically infects a number of mammalian cell lines. The deaminase activity is induced in a mouse cell line that is deficient in this enzyme. The induction of the enzyme in this mutant cell line does not occur in the presence of actinomycin D and the induced enzyme is more thermolabile than the enzyme of the wild-type mouse cell line. Furthermore, a new deoxycytidine deaminase species with a characteristic electrophoretic mobility that is different from that of the host cell enzyme is found in cell extracts prepared from a human cell line infected with herpesvirus. These results strongly suggest that the virus-induced deoxycytidine deaminase is coded by the viral genome. Because a deficiency in this enzyme is conditionally lethal for cells growing in a medium containing 5-methyldeoxycytidine as the sole source of thymidylate, this enzyme can be utilized as a selective marker for selecting mutant cells that have regained deoxycytidine deaminase activity as the result of infection by ultraviolet-inactivated herpes simplex virus.