AbstractA small‐plaque mutant (No. 69) of herpes simplex virus type 2 (HSV‐2) strain 333 has been previously isolated and characterized in this laboratory. This mutant was shown to produce a high ratio of noninfectious to infectious particles when grown at the nonpermissive temperature in hamster embryo fibroblasts [Westmoreland D. and Rapp F. (1976). Journal of Virology 18:92–102]. In this study, we have demonstrated that it is possible to obtain noninfectious stocks of this virus which retain transforming ability in a biochemical transformation assay specific for detection of the HSV gene for thymidine kinase. This mutant contains a DNA genome that has a density identical to the DNA of wildtype virus. Virus and cell DNA synthesis after infection with the mutant at both the permissive and nonpermissive temperature are similar to that observed in cultures infected with the parental virus. Clones of mouse cells biochemically transformed by this virus contain HSV antigens and are presently being examined for oncogen