Microbiological activity of CH_3O-BT (O-p-methoxybenzoylthiamine) and NO_2-BT (O-p-nitrobenzoylthiamine) on L. fermenti was compared with OBT (O-benzolthiamine) ; antagonist of thiamine on L. fermenti because of its strong inhibitory effect on thiamine-uptake by the cells. NO_2-BT showed about one-tenth growth-stimulating potency of thiamine in the broth with or without cysteine, while CH_3O-BT had very slight growth-stimulation with cysteine but no potency for growth without cysteine. By means of bioautography in the broth containing thiamine, CH_3O-BT proved to be inhibitory to growth of L. fermenti but NO_2-BT showed no inhibition at the spot. When these 3 compounds were incubated with phosphate buffer solution (pH 6.0) at 50 or 100℃ for 1 hour, NO_2-BT was more easily converted into thiamine than CH_3O-BT or OBT ; while incubation with the broth at 37℃ for 5 hours, conversion to thiamine was observed to a measurable extentonly in the case of NO_2-BT. NO_2-BT, when incubated with thiamine deficient L. fermenti for 2 hours, was more easily absorbed by the cells than other 2 compounds and in the medium with thiamine, it showed weaker inhibition to thiamine-uptake by the cells than CH_3O-BT or OBT. It was concluded that O-acylthiamine was no more antagonistic to thiamine by reducing its anti-thiamine potency on hydrolysis.