We analyzed herpes simplex virus type 1 (HSV-1) or type 2 (HSV-2) infections in 6 families, to see whether or not a single strain had infected members of each family, analyzing DNA's of viral isolates with restriction endonucleases. We also analyzed 29 isolates from 13 patients to examine if herpetic lesions on different sites of each patient's body had been caused by a single strain. In 4 cases of family infection and 10 cases of individual infection with multiple site manifestations, electrophoretic profiles of digested DNAs of the epidemiologically closely related HSV-1 or HSV-2 isolates were exactly the same. In the other 2 family infections and the other 3 individual infections, there were some differences in size of certain DNA fragments among the isolates which were epidemiologically related to one another. But, these differences did not necessarily indicate that they were distinct strains. Thus, it has been suggested that a single strain spreads among members of a family, and herpetic lesions on different sites of an individual body are caused by the same viral strain in most cases of HSV-1 or HSV-2 infections.