A variety of thiamine-derivatives were determined solely or in combination of the pyrimidine or thiazole moiety of thiamine using Phycomyces blakesleeanus Burgeff (-) strain and the relative activities for the fungus were compared. It was found that the fungus is capable of responding to the pyrimidine moiety of thiamine and other various base-exchanged derivatives of thiamine in the presence of excessive amoun of excessive amount of the thiazole moiety of thiamine. For distinguishing the two, a sulfite cleavage method was shown to be useful.