Bioavailability factors can greatly modify the absorption of dietary iron consumed in different meals by an individual. A greater percentage is generally absorbed of heme iron from animal tissues than of nonheme iron of either animal or plant food. The amount of meat in a meal is the only bioavailability factor known to influence absorption of heme iron. Absorption of iron from the exchangeable nonheme iron pool of a meal is influenced by both enhancing and inhibiting substances or factors. Ascorbic acid, meat, fish, and poultry enhance absorption of nonheme iron, and meals may be classified according to relative bioavailability depending on the content of meat, fish, poultry, and/or ascorbic acid. Some low-molecular-weight organic acids may also increase the bioavailability of nonheme iron. Synthetic metal-chelating agents added to foods and the beverages tea and coffee will inhibit absorption of nonheme iron in a concentration-dependent manner. Wheat bran, soy products, cow's milk, and egg tend to decrease bioavailability of nonheme iron when included in a meal. However, the effect of compounds thought to be responsible for the inhibition in purified form (phytate, fiber, phosphoproteins) is dependent on chemical form and concentration. In some foods there may be as yet unidentified inhibitors or interaction between compounds to inhibit absorption of nonheme iron. Currently available information permits estimation of relative bioavailable iron in a meal.