Mammalian nonheme iron absorption requires reduction of dietary iron for uptake by the divalent metal ion transport system in the intestine. This was thought to be mediated by duodenal cytochrome b (Cybrd1), a ferric reductase enzyme resident on the luminal surface of intestinal absorptive cells. To test its importance in vivo, we inactivated the murine Cybrd1 gene and assessed tissue iron stores in Cybrd1-null mice. We found that loss of Cybrd1 had little or no impact on body iron stores, even in the setting of iron deficiency. We conclude that other mechanisms must be available for the reduction of dietary iron.