Youmans, Guy P. (Northwestern University Medical School, Chicago, Ill.) and Anne S. Youmans. Effect of mycosuppressin on the course of experimental tuberculosis in mice. J. Bacteriol. 84:701-707. 1962.-When mycosuppressin was administered to normal mice prior to infection with virulent tubercle bacilli, no indication of a favorable effect on the course of the infection could be obtained. However, when suspensions of virulent tubercle bacilli were exposed to mycosuppressin for from 1.25 to 3.25 hr prior to infection, survival of the mice was greatly prolonged in comparison with mice receiving untreated tubercle bacilli. Microbial enumeration studies with bovine serum agar medium revealed that the number of viable particles of tubercle bacilli in the mycosuppressin-treated suspensions did not differ significantly from the number in untreated suspensions. When the microbial enumeration studies were conducted using the same medium without serum, the number of viable particles in the mycosuppressin-treated suspensions appeared to be (1/5) to 1/20 of the number in the untreated suspensions. It was concluded that, under the conditions of these experiments, the action of mycosuppressin was primarily bacteriostatic and that this bacteriostasis persisted for a time in vivo following infection of the mice. The possible relation of these findings to the phenomenon of acquired immunity to tuberculosis also is discussed.