Proton nuclear magnetic resonance studies of various mammalian metallothioneins are described. Metallothioneins-1 and 2, isolated from rat and rabbit, and also metallothionein-2, from humans, were investigated. Proton NMR spectra showed that mammalian metallothioneins containing Cd have similar tertiary structures regardless of their sources. On the other hand, metallothioneins containing both Cd and Zn have a slightly different conformation from those containing Cd alone. Almost all methyl resonances of Cd-metallothioneins were assigned to those of Met, Ile, Leu, Val, Thr, and Ala residues by NMR double resonance techniques, pH titration, and the spectral comparison of these metallothioneins. alpha-Proton resonances of Ala residues were classified into two groups, one of which arose around 4.1 ppm, the other around 4.4 ppm. Moreover, the number of alpha-proton resonances around 4.1 ppm and 4.4 ppm agreed with the number of Ala residues in the COOH-terminal and NH2-terminal domains of the metallothionein, respectively. This finding suggests that the chemical shift values of alpha-protons of Ala residues are reflective of the domain structure of the metallothioneins.