For determining vitamin A using glycerol dichlorohydrin (GDH), various methods have thus far been reported, but the author found that none of them showed direct proportionality between the vitamin concentration and optical density. After examining various conditions, an appropriate condition was established for substantiating the proportionality. The values of vitamin A level in normal plasma of Japanese adults were determined in 6 cases by the improved GDH method as well as Carr-Price method. The values determined by both methods were practically in agreement, averaging about 137±30 I.U./100 ml. Cholesterol was shown to have practically no effect on the determination by GDH method.