1. Experiments are presented demonstrating specific inhibition of phage by soluble products of bacteria.
2. The inhibition proceeds more rapidly when the phage and bacterial extracts are incubated at 37° than at ice box temperature.
3. The specificity of the reaction in the instances studied is probably connected with the presence of specific soluble carbohydrates.
4. A reaction is available for the study of the chemistry of bacillary antigens in terms of bacteriophage.