Abstract The defect in Menkes' disease in man is identical to that in Brindled mice. The defect manifests itself in a accumulation of copper in some tissues, such as renal, intestinal (mucosa and muscle), pancreatic, osseous, muscular, and dermal. Hence a fatal copper deficiency results in other tissues (e.g., hepatic). The copper transport through the intestine is impaired and copper, which circumvents the block in the copper resorption, is irreversibly trapped in the above-mentioned, copper accumulating tissues where it is bound to a cytoplasmatic protein with molecular weight 10,000 daltons, probably the primary cytoplasmatic copper transporting protein. This protein shows a Cu-S absorption band at 250 nm, and the copper:protein ratio is increased. Such copper rich protein was found neither in the kidneys of the unaffected mice nor in the liver of the mice that do have the defect. Three models of the primary defect in Menkes disease are proposed.