Derivatives of 3-phenyl-2H-1,3-benzoxazine-2,4(3H)-dione are active against Mycobacterium avium when substituted in position 7 with a methyl. One or two carbonyl groups have to be replaced with a thioxo group. High active derivatives are the compounds without substitution on the phenyl, or those substituted on the phenyl in position 3 or 4 with chlorine, bromine or a methyl. The substitution in position 3 and 4 with two atoms of chlorine lowers the activity. The compounds are active against INH resistant strains. We synthesized other 44 derivatives with a similar structure of the compounds as in the paper but substituted in position 7 with other substituents (chlorine, bromine methoxy). The activity against M. avium was poor. It can be concluded that a new group of compounds with an excellent activity against M. avium has been found.