Intestinal iron absorption was studied in mice with hereditary microcytic anemia (gene symbol mk), an autosomal recessive trait characterized by hypochromia and microcytosis. In vivo studies by means of whole body counting following the intragastric administration of radioiron showed impaired intestinal absorption of iron. Further in vitro studies using the everted duodenal loop technique, demonstrated a defect in the mucosal uptake of iron. Hereditary microcytic anemia should, therefore, provide a valuable model for the study of the mechanisms controlling intestinal iron absorption.