AbstractThe age‐specific prevalence of antibodies to pyrimidine kinase enzymes of herpes simplex virus types 1 (HSV‐1) and 2 (HSV‐2) and of varicella zoster virus (VZV) was measured in serum specimens from 360 persons. Specific inhibition of viral enzyme activity by the serum was used as an indication of the presence of antibodies to the enzyme. For HSV‐1 and HSV‐2 together the overall prevalence of positive sera increased with age and reached about 50% in the older age groups, the major part of the positive sera being HSV‐1 positive. Sera inhibitory to the VZV pyrimidine kinase enzyme were detected only sporadically. Six percent of the tested sera repeatedly inhibited the cytosolar corresponding enzyme of uninfected host cells by an unidentified mechanism. The results of the pyrimidine kinase antibody‐assay test were compared to those obtained by complement fixation (CF) and enzyme immunoassay (EIA) in a separate set of 50 sera. The correlation between the CF and the EIA tests was good in this selected serum set with CF titres from<8 to 64. Twenty‐four percent of the CF positive sera were negative by the pyrimidine kinase antibody assay, while all pyrimidine‐kinase‐antibody‐positive sera also had antibodies detectable by the two other methods. These results confirm and extend earlier observations on the occurrence of antibodies in human sera to HSV and VZV pyri