The growth of L. fermenti 36 and Kl. apiculata was inhibited by triazinothiamine, 4-[(2-methyl-4-amino-5-pyrimidinyl)-methyl]-5-methyl-6-(2-hydroxyethyl)-4,5-dihydro-1,2,4-triazine (I). The inhibition by I was recovered competitively and specifically by thiamine. The inhibition index for L. fermenti was 9480 and that for Kl. apiculata was 41100. The growth of Sacch. carlsbergensis 4228 was stimulated by I of PT-B_1,2-phenyl-4-[(2-methyl-4-amimo-5-pyrimidinyl)-methyl]-5-methyl-6-(2-hydroxyethyl)-2,5-dihydro-1,2,4-triazinium chloride (II) in low concentration but inhibited by them in high concentration. This growthinhibitory effect by I was recovered specifically by addition of pyridoxine but the inhibition by II was not recovered by any vitamin.