Voltammetric studies on the binding properties of the purified metallothionein (MT) fractions isolated from mussel (Mytilus galloprovincialis) digestive gland for cadmium ions were performed. The capacity of MT to complex cadmium and the stability constants of the Cd-MT complex have been determined from the direct titration of MT with cadmium ions in 0.59 M NaCl medium at pH 7.9. Comparing cadmium binding properties of the two isoforms of mussel MT studied denoted as MT10(III) and MT10(IV), it appears that they are distinctly different. In the presence of two different MT isoforms voltammetric determination of cadmium results in specific anodic signals which are observed at E-p = -0.67 V and at E-p = -0.80 V (vs. a Ag \ AgCl electrode) for isoform MT10(III), and isoform MT10(IV) respectively, indicating the existence of two different Cd-MT complexes. This is in agreement with the difference in their calculated apparent stability constants (K = 9 x 10(9) dm(3) mol(-1) for MT10(III) and K = 1.4 x 10(10) dm(3) mol(-1) and K = 2.0 x 10(10) dm(3) mol(-1) for MT10(IV)). The saturation of the two MT10 fractions occurs at different Cd(II) concentrations, and the capacity of MT10(IV) to bind cadmium ions is 2.5 times greater than the capacity of MT10(III). The electrochemical technique, and method for the experimental data assessment presented in this study are suitable for physico-chemical characterization of metallothioneins as the proteins responsible for binding of toxic metals on a cellular level. Using the voltammetric method time dependent changes of the three-dimensional structure of the MT molecule were observed.