3-Phenyl-2H,4H-benz[e][1,3]oxazine-2,4-dione (I) and its derivatives II - XI, substituted on the phenyl ring, can be regarded as a new group of potential antituberculotics. Their activity increases with increasing electron-accepting properties of the substituents. introduction of bromine into the position 6 also positively influences the activity. the compounds are active in vitro against Mycobacterium tuberculosis and M. kansasii. The activity of some of them (VIII, IX0 exceeds that of commercial tuberculostatics used as standards.