Diazinothiamine (I), when 10^<-5>mole was added to the broth with 0.05μg thiamine-HCl, was shown to be inhibitory to the growth of Kl. apiculata but not to be inhibitory to that of L. fermenti. The inhibition of the yeast by (I) was recovered at the presence of 0.10μg thiamine-HCl. Pyrithiamine (II), phenylthiazinothiamine (III), triazinothiamine (IV) and desthiothiamine (V) inhibited cell-uptake of thiamine, when they were incubated with cells of Kl. apiculata or Sacch. cerevisiae and thiamine in the broth, but (I) was shown to be least inhibitory to thiamine-uptake. The antithiamine potency of (I), (IV) and (V) was estimated on thiamine-deficient rats. Oral administration of the compound (I) or (IV) simultaneously with thiamine (I : thiamine-HCl=1000 : 1,IV : thiamine-HCl=500 : 1) resulted in loss of body weights of thiaminedeficient rats. But oral administration of (V) and thiamine (V : thiamine-HCl=1000 : 1) did not cause loss of body weights.