One ml. of aqueous soln. containing 6-60 7 neuraminic acid is mixed with 1 ml. fresh Bial''s reagent (40.7 ml. conc. HC1; 0.1 g. orcinol; 1 ml. 1% FeCl3; water up to 50 ml.), and the mixture heated 5 min. in a paraffin bath at 142[degree]. The red violet pigment formed is extracted with 5 ml. amyl alcohol, the alcohol cooled to 0[degree], centrifugalized, and the amt. of color in the clear alcoholic soln. measured with the step-photometer, with filter No. 53. The conc, of neuraminic acid in the sample is then detd. with the aid of a standard reference curve. The method is accurate even when the cone, of galactose, which gives a green color with Bial''s reagent, is several times that of neuraminic acid in the soln. To determine neuraminic acid in brain tissue, 0.2-0.4 g. of tissue is dried to constant wt. in a desiccator, powdered and extracted in a microsoxhlet apparatus successively with acetone, ether and CHCl3-methyl alcohol. The sugar-rich lipid which contains the neuraminic acid is in the CHC13-methyl alcohol fraction, which is distilled in vacuo, the residue washed with ether, dried, dissolved in 1 ml. water, and neuraminic acid detd. as descr. above. Neuraminic acid is present in the gray matter of the brain, but not in the white. The conc, of neuraminic acid in the dried brain substance is 120-130 mg. %, and in thalamus, 77-90 mg. %. The method is applicable to tissues which have been fixed in formalin.