To determine the blood folacin a fluorometric method is proposed. This includes the following stage-wise operations: liberation of combined forms of folacin under the effect of enzymes contained in the blood investigated; quantification of tyrosine to make a correction for changed fluorescence intensity of this amino acid during oxilation of the solution; measurement of the fluorescence of the substances present in the solution before and after their oxidation with potassium permanganate and calculation of the folacin content. The fluorometric method enables it to greatly speed up the assay by comparison with the microbiological method of determination with the Lactobac. casei as a test-organism, the assay time being cut down from 4-5 days to 4-5 hours without prejudicing the accuracy of the determination.