The use of glycoprotein G (gG) from the herpes simplex virus has made possible an immunoprecipitation test that distinguishes prior infection with herpesvirus type 1 (HSV-I), herpesvirus type 2 (HSV-2) or both. The test is more sensitive than previously available tests. A national study employing this new immunologic test suggests great differences in prevalence of antibodies to each of these two viruses at different ages in different ethnic groups. For example, in the 20-to 25-year age group, about 5% of Caucasians have antibody to HSV-2, while the frequency of antibody to HSV-2 in blacks is about four times that high. This study also suggests a higher prevalence of HSV infection in the population of women of childbearing age than was previously estimated with less sensitive testing methods. Since HSV disease in newborns is closely related to the frequency of herpes simplex infection in the mother, estimates of the likelihood of nosocomial infection with HSV in newborns must take these new data into account. The specificity of the new test was not discussed.