THE susceptibility of mammalian cells to virus infection has been studied in many virus–host systems. In certain instances1–6, the comparison of cells which do not permit productive infection with those that do has made it possible to identify the host factors which allow entry of the virus and support its subsequent growth. In the case of poliovirus the use of somatic cell hybrids between permissive and non-permissive cells has led to the assignment to human chromosome 19 of a gene coding for virus susceptibility7. Although cells from a wide variety of organisms, including many rodents and man, are susceptible to lytic infection by herpes simplex virus type 1 (HSV-1) resistance to HSV-1 infection has been reported in certain Chinese hamster cell lines8–10, in dog kidney cells11, and in a line of rat cells12. In these cell lines HSV-1 is either non-infectious or exhibits virus titres several orders of magnitude lower than in Syrian hamster or rabbit cells. The cellular factors involved in this resistance to infection and in the genetic control of susceptibility to HSV-1 have not yet been identified.