1. The thermal inactivation time at 41.5°C. of the H. F. and Frank strains of herpes simplex virus, under the conditions described, was 80 hours.
2. A strain of herpes virus recently recovered from a patient treated with a physically induced fever had a thermal inactivation time of 50 hours at 41.5°C.
3. The neurotropic factor of the H. F. and Frank strains of virus was more resistant to a temperature of 41.5°C. than the dermotropic factor. There was little difference in the thermostability of these two factors of the Go strain.