Until a very short time ago it was generally accepted that typical tuberculin hypersensitiveness in guinea pigs, as elicited by skin reactions, could not be produced without the presence of living tubercle bacilli in the body of the animal. This had been the conclusion of practically all workers who had seriously studied the subject, and, indeed, was the experience of one of the writers in a study of tuberculin hypersusceptibilities and anaphylaxis published in 1921 (1). It seemed at that time as though one of the most important factors in the elucidation of the biological changes which take place in the infected animal would be a more precise knowledge of the conditions which determined tuberculin hypersensitiveness. The task we set ourselves, therefore, was an attempt to simulate, with dead tubercle bacilli or their products, conditions analogous to those produced in the guinea pig in the course of infection with living bacilli.