The relative availability of water-soluble vitamins to free vitamins in a white bread diet consumed by Japanese male and female subjects was determined. The subjects, 9 female Japanese college students in experiment 1, and 7 male Japanese and 5 female Japanese college students in experiment 2, consumed the test diet with or without a water-soluble vitamin mixture for five consecutive days, and the water-soluble vitamin levels in a 24-h urine sample were measured. The ratio of the urinary excretion rate for each water-soluble vitamin in the test diet to that in the water-soluble vitamin mixture was determined as the relative availability. The relative availability of each vitamin was as follows: B₁, 55%; B₂, 50%; vitamin B₆, 85%; niacin, 60%; pantothenic acid, 70%; folate, 50%; biotin, 85%; and C, 95%.