Difference in plaque-forming ability in chick embryo cells was shown to be a reliable marker for differentiating herpes virus types 1 and 2 when original or low-passaged isolates of the viruses were used. Type 2 viruses produced large plaques at titres equal to those obtained in rabbit kidney cells, while type 1 viruses failed to produce plaques or produced small plaques at a low plaquing efficiency. Naturally occurring populations of type 1 virus were found to contain a small proportion of virus which produced plaques in chick embryo cells, and this proportion increased with passage of the virus in tissue culture. The plaques produced by type 1 virus in chick embryo cells were morphologically distinct from the plaques produced by type 2 virus. The type 1 variants which plaqued in chick embryo cells could be plaque-purified, and the progeny viruses maintained antigenic and biological similarities to the parent type 1 virus except for the relative resistance to the DNA inhibitor ara-A.