This study was performed to determine whether synthetic ascorbic acid (AA) alone or in a natural citrus extract containing bioflavonoids, proteins, and carbohydrates was more bioavailable to human subjects. The effect of a single 500-mg ascorbate dose of the two forms and a placebo citrus extract on plasma ascorbate was examined in eight fasting subjects. A comparison of the areas under the plasma concentration-time curves showed that the citrus extract was 35% more absorbed than AA (p less than 0.001) and was more slowly absorbed than AA (p less than 0.001). In six ascorbate-saturated male subjects the ascorbate in the citrus extract produced a greater ascorbate excretion than AA alone in 24-h post-dose urine (p less than 0.05). Citrus extract ascorbate was less excreted than AA (p less than 0.05) in 12 nonsaturated subjects. Ascorbate in the citrus extract was found to be more bioavailable than AA alone in human subjects.