In a blind study, 21 clinical isolates of herpes simplex virus which had been typed using differential growth on guinea pig embryo versus chicken embryo cells were tested for the presence of the viral deoxyribopyrimidine triphosphatase. In all isolates of type 1, the triphosphatase was present in the nuclei of the infected cells, while none of the HSV-2 isolates induced a nuclear enzyme. In all isolates there was a complete correlation between the presence of nuclear deoxyribopyrimidine triphosphatase and sensitivity to 0.7 microgram/ml of E-5-(2-bromovinyl)-2'-deoxyuridine. The study suggests that the type-specific distribution of the triphosphatase is of general validity, including clinical isolates of herpes simplex virus, and could be used as a type-specific enzyme marker.