The in vivo properties of four host range temperature-sensitive (ts) mutants of herpes simplex virus type 2 (HSV-2) were correlated with known in vitro characteristics. Mutants and parental virus were inoculated into newborn and weanling mice and weanling hamsters by intracranial and subcutaneous routes, and into weanling rabbits by corneal scarification. In all cases the pattern of attenuation of mutants in vivo correlated with their stability in vitro at 39°. The most attenuated mutant (mutant 69) was also the most consistent in its inability to induce cytopathic effects (CPE) in vitro or to replicate under nonpermissive conditions. Conversely, the most virulent mutant (mutant 41) was the least stable under nonpermissive conditions