In working on the neutralizing powers of the serum of rabbits immunized against the agent of herpes, the writers noticed some years ago (1) that salt solution suspensions of this material taken from the brains of rabbits dead of the disease rapidly lost virulence, upon incubation at 37.5°C. There was a distinct diminution of infectiousness within two or three hours, and often complete loss of virulence within five or six hours, the time period varying with the densities of the suspensions. Suspensions in which the amount to be inoculated into a rabbit (0.1 to 0.25 cc.) contained not more than three or four fatal doses—that is, dilutions of 1:16,000 according to the potency of the virus—were often completely inactivated within four or five hours, whereas heavier suspensions would last correspondingly longer, probably because of the presence of minute particles of brain material within which the herpetic agent was protected to some extent from the deteriorating influences active in the salt solution.