The dose-dependent inhibitory effect of sodium phytate on iron absorption was studied in man by serving wheat rolls containing no phytates and rolls to which various amounts (seven dose levels between 2 and 250 mg expressed as phytate phosphorus) were added just before serving. Fe in the two kinds of rolls was labeled with two radioisotopes of Fe (55Fe, 59Fe) and the rolls were served on alternate days. The inhibition of Fe absorption was strongly related to the amount of phytate added; 2 mg inhibited absorption by 18%, (p less than 0.001), 25 mg by 64% (p less than 0.001), and 250 mg by 82% (p less than 0.001). The addition of ascorbic acid significantly counteracted the inhibition whereas the corresponding effect of meat was less well defined and only seen at the highest phytate level. The marked inhibition of Fe absorption by phytates and the significant counteracting effect of ascorbic acid have wide nutritional implications.