Previously, hepatic iron overload resembling that in hereditary hemachromatosis (HH) has been found in beta 2-microglobulin knockout (beta 2m-/-) mice. We have now characterized iron metabolism in beta 2m-/- mice. The mutant mice fail to limit the transfer of iron from mucosal cells into the plasma. Transferrin saturation is abnormally high. Pathologic iron depositions occur predominantly in liver parenchymal cells. Reconstitution with normal hematopoietic cells redistributes the iron from parenchymal to Kupffer cells, but does not correct the mucosal defect. We conclude that (a) iron metabolism is defective in the gut mucosa as well as the liver of beta 2m-/- mice; and (b) a beta 2m-dependent gene product is involved in iron homeostasis. Recently, a novel gene of the major histocompatibility complex class I family, HLA-H, has been found to be mutated in a large proportion of HH patients. Our data provide functional support for the proposed causative role of HLA-H mutations in HH.