Isothermal titration calorimetry was firstly applied to investigate the metal-binding properties of metallothionein. Thermodynamic data of the replacement of metallothionein (Zn7MT) and the reconstitution of thionein (apoMT) with Pb(II) ion at pH 4.70 have been examined. The UV and CD spectrometries were also used to identify the products. The product of the reaction of Pb2+ with Zn7MT (reaction 1), Pb7MT, has a similar structure with Zn7MT, while a new lead-MT complex, Pb7MT', is formed in the case of reconstitution of apoMT (reaction 2). The thermodynamic parameters of the reactions for MT binding each mole of lead(II), Delta H-1=-25.2 kJ mol(-1), Delta G(1)=-28.7 kJ mol(-1), K-1=1.3x10(5), Delta S-1=11.6 J mol(-1) K-1 and Delta H-2=-17.8 kJ mol(-1), Delta G(2)=-32.4 kJ mol(-1), K-2=5.3x10(5), Delta S-2=48.8 J mol(-1) K-1, demonstrate that both reactions are spontaneous, exothermic, and entropy-increasing precesses. The contributions of relative interactions in both reactions to enthalpy and entropy changes are discussed.