The effects of proteins and protein digestion products on iron bioavailability were studied. Iron dialyzability, under simulated gastrointestinal conditions, in the presence of selected proteins and fractionated protein digestion products, was used as a measure of iron bioavailability. Dialyzability was high for bovine serum albumin and beef, intermediate for egg albumin, and low for soy flour, gelatin, casein, soy protein isolate, and gluten. Low molecular weight digestion product fractions from bovine serum albumin and beef enhanced iron dialysis; similar fractions from casein and soy protein isolate did not affect iron dialysis. The influence of proteins on iron bioavailability appears to be related to the affinity of undigested or partially digested protein for iron and to the stability of small molecular weight soluble iron complexes formed from protein digestion products.