Restriction endonuclease fragment length polymorphism (RFLP) is useful for the epidemiological study of herpes simplex virus type 1 (HSV-1). We report here the identification of a major BglII RFLP variant of HSV-1, designated BgK L , found in 27.0% of 636 HSV-1 clinical isolates. We have also established its geographic distribution in Japan. BgK L has an unusually large BglII K fragment. SalI cleavage analyses showed that 97% of BgK L variant isolates lack both the SalI C-J and the F-J cleavage sites and have an unusually large SalI D or E fragment, and 91% of the BgK L variants lack both SalI G and H fragments. Furthermore, 96% of BgK L isolates have an unusually small KpnI M fragment. Therefore, BgK L is a marker for these five mutations in most HSV-1 isolates and is a useful HSV-1 RFLP marker. The BgK L variant was found in 59% of HSV-1 isolates from Shikoku Island, 44% of HSV-1 isolates from the Chugoku region of Honshu Island, 31% of HSV-1 isolates from Kyushu Island, 0% of HSV-1 isolates from Okinawa Island, 49% of HSV-1 isolates from Osaka, 27% of HSV-1 isolates from Shiga, 13% of HSV-1 isolates from the Chubu Region, and 9% of HSV-1 isolates from the Tohoku Region of Honshu Island. Differences in the frequency of BgK L between the ShikokuChugoku-Osaka area (49%) and Kyushu, between Kyushu and Okinawa, between the Shikoku-Chugoku-Osaka area and Shiga, and between Shiga and Tohoku are all statistically significant. The BgK L frequency decreases in a geographical gradient suggest that this HSV-1 variant was dispersed from Shikoku to the surrounding regions and then to more distant regions. The BgK L frequency in Tokyo was similar to the nationwide average. These are the first data to suggest a geographic and demographic dispersion pattern of HSV-1. Implications for the epidemiology and diversification of HSV-1 are discussed.DNA restriction endonuclease cleavage patterns and restriction fragment length polymorphism (RFLP) are useful for epidemiological studies of herpes simplex virus type 1 (HSV-1) strains (7,14,15,25,28,29). RFLP profiles have been used to differentiate HSV-1 clinical isolates from epidemiologically unrelated individuals and to determine whether viruses isolated from epidemiologically "connected" individuals are identical (1-3, 5, 9, 12, 13, 16, 18, 19, 23, 26, 28, 36). The data indicate that the genomic DNAs of HSV-1 isolates have accumulated enough base substitutions to allow different isolates to be distinguished. HSV-1 polymorphism also has been observed by differences in the electrophoretic patterns of virion structural proteins of different HSV-1 strains (17,21,28). The lifelong latency and reactivation of HSV-1 infection probably facilitates the accumulation of random mutations and diversification of 37,38). HSV-1 isolates with distinct base substitution mutation(s) at restriction endonuclease cleavage sites are designated RFLP variants in here.It is not known whether an HSV-1 RFLP variant isolated at a certain frequency in a particular population is due to an RFLP variation that aro...