A micro adaptation of the thiochrome technique is described. The lower limit is 0.001 [mu]g B1 at which the variation between replicates is [plus or minus] 20%. Above 0.05 [mu]g the variation is [plus or minus] 3%. An oval section capillary tube of bore 1 X 3 mm. and 0.12 ml. capacity is used to hold the test soln. for measurement in the Spekker Photoelectric Fluorimeter. Methyl alc. is omitted during oxidation to thiochrome and the strength of K fer-ricyanide adjusted for the soln. under test to give maximum thiochrome fluorescence. For standard aneurin soln. this is 0.005% and for the cereal extracts under test 0.25%. Figures which show the interaction of the several reagents used in the thiochrome method are given.