PART I. The chemical identification of vitamin C in human urine by isolation of the 2 :4- d.ínitrophenylhydrazine derivative of dehydroascorbic acid. From 12 1. of urine, 20 mg. of the pure 2:4- dinitrophenylhydrazine derivative of dehydroascorbic acid were isolated after repeated adsorbtión on aluminium oxide and crystallization from acetic acid and from acetone or acetone -alcohol. The substance was identified by crystalline tor 0 P.P. and mixed M.P., colour reaction with sulphuric acid and with sodium hydroxide, absortpion in the visible spectrum and distribution between two immiscible solvents. A second hydrazone, similarly adsorbed, has been isolated, but not yet identified. PART II. The quantitative determination o ascorbic acid in human urine. The various forms in which it may be found to occur. it is confirmed that mercuric acetate, in suitable concentration, removes from urine certain subtances including thiosulphate, which interfere with the indophenol titration method for the estimation of ascorbic acid. The Ilse of "hexoxidaoe," from cauliflower shows that reducing substances other than ascorbic acid are still present. Prolonged treatment with H₂S increases the indophenal-reducing power of urine, and it is confirmed that acid hydrolysis has a similar effect. Hydrolysis followed by reduction produces the maximum increase. Enzyme experiments show that both these Increases are due in part to production of extra ascorbic acid (if the enzyme is specific) which is probably present in urine partly in the "reduced" form, partly as dehydro-ascorbic acid, and partly as non-reducing hydrolysable derivatives of these substances. Associated with ascorbic acid is another indophenol -reducing substance, closely resembling it in projperties, and also existing in non -reducing "oxidized" and "combined" forms. A method of estimating ascorbic acid in urine is suggested.