A newly reported spontaneous mutation in mice resembles human atransferrinemia. Both the human and murine forms have a Mendelian recessive mode of inheritance, and both are early onset lethal diseases in which homozygotes have refractory iron-deficient hypochromic anemia. Their plasma transferrin levels are less than 4 mg/dl, and extensive deposits of iron are found in the livers and other organs of afflicted individuals. Although heterozygotes are not anemic, they have half normal levels of circulating transferrin, and they become siderotic late in life. In the mouse, weekly injections of mouse or human serum, or of highly purified transferrin, permit survival and alleviate the anemia. The pattern of inheritance, clinical features, time course and distribution of parenchymal iron, and nature and stability of the endogenous transferrin are presented. The data indicate that the gene hpx produces wholly normal strain-specific transferrin but in small amounts. The hpx gene is closely linked with the transferrin locus, Trf, on chromosome 9.