IN earlier work on cocarboxylase (thiamine diphosphate) as a phosphate carrier, an unknown spot was often found, not reported in the previous publication1, when extracts from liver were chromatographed in order to isolate thiamine diphosphate from other phosphate compounds. This unknown spot gave, like thiamine diphosphate, a blue fluorescence in ultra-violet light after having been oxidized with potassium ferricyanide in alkaline solution. It also, like thiamine diphosphate, had biological activity when tested in a Warburg apparatus with washed yeast. In 1949 Velluz et al. 2 reported their work on synthetic thiamine phosphate, thiamine triphosphate, and its relation to thiamine diphosphate. When comparing this compound on a chromatogram with the unknown compound referred to above, I found them to behave in exactly the same manner. the same time Rossi-Fanelli et al. 3 confirmed the occurrence of thiamine triphosphate in rat liver.