To analyze the mechanisms for in vitro emergence of the syncytial variants of herpes simplex virus type 1 (HSV‐1), several cell lines were infected with a mixture of equal amounts of two HSV‐1 variants, one syncytial and the other non‐syncytial, and changes in their relative abundance were monitored during passage. With a combination of two variants of the Miyama strain of HSV‐1, the syncytial variant became dominant during passage in Vero, RK‐13 and FL cells. On the other hand, the ratios of the two variants remained around 1:1 during the passage in HEp‐2, MGC and HEL cells. In another set of variants of the SKO strain of HSV‐1, the outcomes were different from those of the Miyama strain in the FL, MGC and HEp‐2 cells. The ratios of the two variants remained around 1:1 during passage in FL cells, while the non‐syncytial variant became dominant during passage in MGC and HEp‐2 cells. In addition, we examined the effects of a complement and interferon‐β (IFN‐β) on the outcome of the selection. As a result, the complement slowed the selection of a syncytial variant, whereas IFN‐β facilitated it.