Chemical, spectroscopic, and structural studies have established the metallothioneins (MTs) to be a widely occurring family of polypeptidic bioinorganic structures. They are distinguished by an extremely high metal (Zn, Cd, Cu) and Cys content and by the arrangement of these components in metal-thiolate clusters. By structural criteria the MTs have recently been subdivided into three classes (Fowler et al., Experientia Suppl. 52, 19-22, 1987). Class I MTs include mammalian MTs and related forms. Class II MTs display no such relationships, and Class III MTs are atypical polypeptides made up of repetitive gamma-glutamylcysteinyl units. Amino acid sequences of over 50 MTs are now known. In mammals, over 55% of the residues, including the 20 Cys, are conserved. Mammalian MTs are genetically polymorphous. Thus, in human tissues and cell lines closely related structures of ten functional isoMTs have been determined either by amino acid or nucleotide sequencing. A comparable degree of polymorphism also exists in the rabbit. Mammalian MTs have been inferred to bind a total of seven bivalent metal ions (Me) through thiolate coordination in two separate clusters, i.e., Me(II)3(Cys)9 and Me(II)4(Cys)11. This two-cluster model has now fully been confirmed by the spatial structures of rat MT-2 and rabbit MT-2a determined by 2D NMR spectroscopy in aqueous solution.