Deoxythiamine (DOB_1) is a new type of vitamin B_1 antagonist (Vitamins 36,326. 1967) . S-Benzoyldeoxythiamine (BDOB_1) was synthesized from DOB_1 by applying the method of benzoylation. of B_1 and its B_1 antagonistic effect was compared with that of DOB_1 on microorganisms. BDOB_ itself showed no inhibitory effect on the growth and on B_1 uptake of L. fermenti and Kl. apiculata while the coexistence of cysteine led to remarkable B_1 antagonistic effect. It was observed that BDOB_1 was changed by cysteine to DOB_1 in the medium which resulted in inhibitory effect.