The cause of marked inhibitory effect of bran on absorption of dietary nonheme iron was studied in man by double-radioiron technique. Washing bran with hydrochloric acid but not with water removed inhibitory factor(s). Inhibition was almost restored by reconstituting phytate level. Removal of phytates in bran by endogenous phytase significantly increased absorption of iron. Removing, by washing with water, phosphates formed from phytates during enzymatic dephytinization led to a bran fraction with only a small remaining inhibitory effect on iron absorption. Half the iron in bran is in the form of monoferric phytate, which is well-absorbed. When potassium and magnesium phytates were added in amounts present in bran, the same inhibitory effect on iron absorption was seen. Although there appear to be other factors in bran that partly explain the inhibition, phytates are the main cause of the inhibitory effect of bran on iron absorption.