Benzaldehyde- and salicylaldehyde-S-benzylisothiosemicarbazones show a moderate to high in vitro activity against Mycobacterium tuberculosis, Mycobacterium avium and two strains of Mycobacterium kansasii. Benzaldehyde-S-4-bromobenzylisothiosemicarbazone and salicylaldehyde-S-4-chlorobenzylisothiosemicarbazone have the most promising antimycobacterial properties.