The copper(I) and silver(I) binding properties of the β fragment of recombinant mouse metallothionein 1 have been studied by electronic absorption and circular dichroism spectroscopy. When possible, the stoichiometry of the species formed was confirmed by electrospray mass spectrometry. The behaviour observed differs from that reported for the native protein. Titration of either Zn 3 -βMT at pH 7 or apo-βMT at pH 3 with Cu + leads to the formation of species having the same stoichiometry and structure: Cu 6 -βMT, Cu 7 -βMT and Cu 1 0 -βMT. In the first stage of the titration of Zn 3 -βMT with Cu + at pH 7 one additional species of formula Cu 4 Zn 1 -βMT was detected. In contrast, the titration of Zn 3 -βMT at pH 7.5 and of apo-βMT at pH 2.5 with Ag + proceeds through different reaction pathways, affording Zn x Ag 3 -βMT, Ag 6 -βMT and Ag 9 -βMT or Ag 3 -βMT, Ag 6 -βMT and Ag 9 -βMT, respectively. The CD envelope corresponding to species with the same stoichiometric ratio, Ag 6 -βMT and Ag 9 -βMT, indicates that they have a different structure at each pH value. On the basis of the differences observed, the postulated similarity between copper and silver binding to metallothionein may be questioned.