The way in which meat and fish act to promote the absorption of nonheme iron in food is not known. The present paper is a report of the results of a series of studies aimed at obtaining some insight into the mechanism of action of meat and other animal proteins on the absorption of food iron. Beef, fish, chicken and calf thymus all increased the iron absorption to about the same extent. Neither egg albumin, cysteine or a water extract of beef did, however, affect the absorption of food iron. Beef increased the absorption of a solution of inorganic iron given without food only when the iron salt was trivalent or when sodium phytate was added to the solution. It was concluded that meat acts by counteracting luminal factors that inhibit iron absorption. The most probable mechanism for this action is formation of a luminal carrier which transports the iron to the mucosal cell membrane.