The ability of various animal proteins to enhance the absorption of dietary nonheme iron was evaluated by performing multiple radioiron absorption measurements in 70 volunteer subjects. Protein equivalent substitutions of nine animal foods were made in two basic test meals. The first was a standard meal of high iron availability (mean absorption, 8.3%) containing beef muscle as the animal protein. The second was a semisynthetic meal of low iron availability (mean absorption, 1.4%) containing ovalbumin as the protein source. Two categories of animal protein were defined. Substitution of beef, lamb, pork, liver, fish, and chicken for the egg ovalbumin in the sannisynthetic meal resulted in a significant, 2-fold to 4-fold increase in iron absorption whereas no increase was observed with milk, cheese, or egg. Reciprocal findings were obtained when these foods were substituted for the beef contained in the standard meal. All sources of animal proteins are not equivalent in their effect on nonheme iron absorption.