A study of Cd complexation with the peptide Lys-Cys-Thr-Cys-Cys-Ala thionein fragment [56-61] MT I (FT) was performed in the differential pulse anodic stripping voltammetric DPASV) mode. Amperometric titrations of FT with cadmium standard solution were performed in 0.59 mol dm(-3) NaCl at pH 7.9, and at a constant temperature of 25.0 degreesC. The signal of FT without cadmium has the peak potential at E-p(FT) = -0.52 V (vs. an Ag \ AgCl electrode). As the titration with CdCl2 proceeds, a signal related to CdFT complex appears at the potential E-p(CdFT) = - 0.67 V. When the available complexing sites on FT are saturated the signal of ionic Cd appears. From the amperometric titration data the capacity of FT to complex Cd (C,) and the apparent stability constant of the CdFT complex (K ') using a modified van den Berg-Ruzic-Lee method were determined. The mean value of the stability constant corrected for the partition of the hydrated Cd-2 (+) ion within the ionic cadmium concentration is 5.3 x 10(10) dm(3) mol(-1). In addition, titration of the constant Cd2 + concentration with FT was performed. Due to the excess of cadmium over the FT and vice versa the existence of complexes of different stoichiometry are proposed to cause the poorly resolved signal.