Principle: Thiamine is extracted with a 25 % solution of KCl in 0.1 M HCl at room temperature. This extraction is found to be complete even in those instances in which an absorbent for the vitamin is present or is produced in the sample during extraction. The thiochrome resulting from oxidation of the filtrate 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 a standard solution. The intensity of fluorescence is measured photoelectrically and the fluorescence value of a blank test is subtracted from that of the sample extract. The use of the HCl blank results in a considerable saving in time required for analysis. There is little or no interference from sample material and no indication that the samples contain substances capable of quenching thiochrome fluorescence under the conditions of the method. There it is sufficient for measurement and calculation to outline in the assay directions the use of one external or outside standard.