Principle: In order to free the thiamine from the natural ester and protein bonds, the material to be examined is digested with sulphuric acid and subsequently treated with a phosphatase preparation. In order to exclude foreign fluorescing material the thiamine is separated from the extract by adsorption. The thiochrome resulting from oxidation with potassium ferricyanide in alkaline solution is extracted with isobutyl alcohol. The intensity of the blue fluorescence of the isobutyl alcohol extract is compared with that of the standard solution. The intensity of fluorescence is measured photoelectrically and fluorescence value of a blank test is subtracted from that of the sample extract.