Multinuclear 1 and 2 dimensional magnetic resonance methods have been used to investigate the structures and metal binding properties of metallothioneins (MTs) isolated from several different sources. 113Cd NMR studies have unambiguously shown that the 7 g-atoms of Cd2+ bound per mole of the mammalian MT are located in two separate metal clusters, one containing 4 metal ions and the other, 3 metal ions. In the invertebrate (Scylla serrata) MT, similar studies have revealed that the 6 g-atoms of bound Cd2+ are distributed in two distinct 3-metal clusters while in Neurospora MT, the 3 g-atoms of bound Cd2+ are arranged in a pseudo 3-metal cluster. With the exception of one of the Cd2+ sites in this latter cluster, all the Cd2+ ions are tetrahedrally coordinated to four cysteine thiolate ligands with single cysteinyl sulfurs bridging adjacent metals. These conclusions are based on the 113Cd chemical shift data and a detailed analysis of the observed 113Cd-113Cd scalar couplings by both homonuclear decoupling and 2D techniques. In addition, the 113Cd NMR studies have revealed significant differences in the affinity of different metal ions for the two mammalian metal clusters. For the 3-metal cluster, the affinity is found to decrease in the order Cu+ greater than Cd2+ greater than Zn2+ with Cd2+ greater than Zn2+ for the 4 metal cluster and Cd2+ (4-metal cluster) greater than Cd2+ (3-metal cluster). The 113Cd NMR data are currently being integrated with 500 MHz 2D 1H and 1H-113Cd chemical shift correlated multiple quantum data sets to more completely define the structural arrangement of the metal clusters in the tertiary structure of these proteins.