Abstract At pH 4, the non-fluorescent 1,2-naphthoquinone-4-sulfonic acid is quantitatively reducedby ascorbic acid to 1,2-dihydroxynaphthalene-4-sulfomc acid, which intensely fluoresces at λ ex = 330 nm and λ f1 = 465 nm. This permits to determine ascorbic acid in aqueous solution and in oxalated blood plasma. In the latter case, protein is removed by precipitation with metaphosphoric acid. When compared with other techniques of measuring the reducing power of vitamin C, this method provides far better sensitivity and shows good reproducibility (standard deviation ±1.8%). It is easy to perform and was found very satisfactory for the determination of ascorbic acid in the blood of patients having subnormal levels of this vitamin. The limit of sensitivity is 0.1 μg per ml of blood plasma.