5, 6-Dihydro-2H-1, 3-thiazin-4 (3H)-ones 2–10 related to the natural antituberculytic Mycobacidine 1, have been synthesised. Some of the compounds of types 2, 3 and 5 proved highly active in vitro against Mycobacterium tuberculosis typ. humanus (H37Rv), but the in vitro activities are diminished or suspended in the presence of biotine.