Earlier studies have shown that herpes simplex virus type 1 (HSV-1) activated protein kinase R (PKR) but that the product of the product of the γ134.5 gene binds and redirects the host phosphatase 1 to dephosphorylate the a subunit of eukaryotic translation initiation factor 2 (eIF-2α). In consequence, the γ134.5 gene product averts the threatened shutoff of protein synthesis caused by activated PKR. Serial passages of Δγ134.5 mutants in human cells led to isolation of two classes of second-site, compensatory mutants. The first, reported earlier, resulted from the juxtaposition of the a promoter of the Us12 gene to the coding sequence of the Us11 gene. The mutant blocks the phosphorylation of eIF-2α but does not restore the virulence phenotype of the wild-type virus. We report another class of second-site, compensatory mutants that do not map to the Us10-12 domain of the HSV-1 genome. All mutants in this series exhibit sustained late protein synthesis, higher yields in human cells, and reduced phosphorylation of PKR that appears to be phosphatase dependent. Specific dephosphorylation of eIF-2α was not demonstrable. At least one mutant in this series exhibited a partial restoration of the virulence phenotype characteristic of the wild-type virus phenotype. The results suggest that the second-site mutations reflect activation of fossilized functions designed to block the interferon response pathways in cells infected with the progenitor of present HSV.