A modified method has been described for the estimation of ascorbic acid (AA) and dehydroascorbic acid (DHA) in blood and plasma. DHA is practically absent in the blood of normal human beings. On the other hand, diabetic patients have a persistently high blood DHA level. The DHA from diabetic blood has been isolated as the 2,4-dinitrophenylhydrazone derivative and identified by thin-layer chromatography and spectrophotometry. © 1979.