SummaryThe above work shows that mustard gas added to brain suspensions from animals infected with fixed virus gives a vaccine which by the method of testing used is better than that produced when the same brains are inactivated with phenol, about equal to that when the inactivation is by ultraviolet light, and almost as good as active fixed virus. The vaccine is very stable at refrigerator temperatures, and the method of preparation is simple and requires no unusual laboratory equipment.