Vertebrate metallothioneins are found to contain Zn(II) and variable amounts of Cu(I),in vivo, and are believed to be important for d 10 -metal control. To date, structural information is available for the Zn(II) and Cd(II) forms, but not for the Cu(I) or mixed metal forms. Cu(I) binding to metallothionein-1 has been investigated by circular dichroism, luminescence and 1 H NMR using two synthetic fragments representing thea- and the b-domain.