Herpes simplex viruses have been divided into two types. These types may be differentiated by serological tests (Shubladze et al. 1960; Plummer, Waner & Bowling, 1968) and by the character of the lesions produced on the chorioallantoic membrane (CAM) of fertile hens' eggs (Parker & Banatvala, 1967) and in tissue culture (Lowry, Melnick & Rawls, 1971). It has been suggested recently that the serological classification into type 1 and type 2 may be too rigid and that a whole spectrum of variants may exist (Roizman et al. 1970).