The effect of varying concentrations of ascorbic acid on the absorption of iron from a soy-based infant milk formula containing 6 mg iron/100 g was examined in 64 adult Indian females using the extrinsic radioactive tag method. The corrected geometric mean absorption from the basic soy formula was only 1.8%. Addition of ascorbic acid in a concentration of 40 mg/100 g, did not significantly increase absorption (3.3%; t = 1.8, p greater than 0.07) but raising the concentration to 80 mg/100 g did so (6.9%; t = 2.4, p less than 0.02). No further significant increase was noted when the concentration of ascorbic acid was increased to 160 mg/100 g (7.7%; t = 0.4, p greater than 0.7). The inhibitory effect of soy on iron absorption was further demonstrated by a direct comparison between the soy-based formula and a similar product based on cows' milk. The comparison was made at two concentrations of ascorbic acid. At 40 mg/100 g the geometric mean iron absorption from the soy formula was 2.4% compared with 5.3% from the milk formula (t = 2.8, p less than 0.02), while the corresponding values at 80 mg ascorbic acid/100 g were 7.2 and 19.5%, respectively (t = 3.4, p less than 0.02). The present results confirm the marked inhibitory effect of soy protein on iron absorption and calculations from the absorption figures suggest that such formulas should contain at least 12 mg/100 g iron together with ascorbic acid in a molar ratio of approximately 4:1 if they are to be adequate in terms of iron nutrition.