Menkes' disease is a recessive X-linked disturbance of copper metabolism, resulting in accumulation of copper in several extra-hepatic tissues including the placenta. Metallothionein (MT) is a low-molecular weight protein with a high affinity for group II metal ions, such as copper. Its synthesis is induced by the presence of the ions. The aim of this study was to investigate the pattern of the MT immunoreactivity in placental tissue obtained from women at-risk of Menkes' disease in order to examine whether the MT occurrence and distribution may reflect the copper content. Placental tissue from six women with a family history of Menkes' disease, from 4 women without a family history, and from 2 hydatiform moles was studied. Positive MT immunostaining was found to be independent of the length of fixation, whether the tissue samples were fixed in 4% buffered formaldehyde or Bouin's fixative. The avidin-biotin-complex (ABC)-technique was used. The copper content was measured by neutron activation analysis (NAA). In all placental tissue sections positive MT immunostaining appeared only in the trophoblast and only in proliferating cells. In placental tissue sections obtained from foetuses and children affected by Menkes' disease an additional MT immunostaining appeared in the Hofbauer cells of the chorionic villi. This staining was associated with an increased content of copper as measured by NAA. We conclude that the immunohistochemical demonstration of MT reflects the copper content and may be useful in pre- and postnatal diagnosis of Menkes' disease.