Menkes' kinky hair disease, a lethal X-linked recessive trait, is characterized by abnormal copper accumulation in several non-hepatic tissues. The level of many copper enzymes is severely reduced, leading to damage of the connective and nervous tissues of the patients. Cultured skin fibroblasts from Menkes' patients retain more copper then normal controls, and the excess metal is bound to metallothionein. Low doses of copper in the media induce MT gene transcription in Menkes' but not in normal cells. Transfection experiments using a plasmid containing the mouse MT-I promoter fused to the enzyme chloramphenicol acetyl transferase show that the activation of the mMTI promoter is in trans. Two other effects are observed in Menkes' cells: (a) two heat-shock like proteins are synthesized in response to low doses of copper in the growth medium, and (b) Menkes' cells are more sensitive then normal fibroblasts to copper toxicity. Our interpretation of these results supports a model for a defect in one or more steps in copper metabolism or transport.