46 strains of Mycobacterium tuberculosis were studied as to their susceptibility to isoniazid (INH) or to INH + prothionamide (PTH) in the presence or absence of dapsone (DDS) by microdilution transfer plate technique. Synergy with DDS as defined by Berenbaum in 1978 was seen in 78% with INH and in 91% with INH + PTH. It concerned the minimal inhibitory concentration, whereas the minimal bactericidal concentration (MBC) was unaffected in the case of INH and it was increased in only 52% with INH + PTH. The latter MBC proportion mentioned would be decreased to roughly 16% accepting an only twofold decrease in the MBC as possibly due to dilution errors. The possible effect of dimethyl sulphoxide on the reported results is discussed.