The interaction of serum albumin with amaranth was studied at the mercury electrode with different electrochemical methods such as cyclic voltammetry and linear sweep voltammetry in this paper. In pH 3.0 Britton-Robinson (B-R) buffer solution, amaranth showed an irreversible voltammetric reductive peak at -0.12 V (vs. SCE). After the addition of human serum albumin (HSA) into the amaranth solution, the reductive peak currents decreased without any shift of the peak potentials. Electrochemical equations suitable for studying the interaction of irreversible electro-active molecules with HSA were proposed. The electrochemical parameters of the interaction system were calculated in the absence and presence of HSA and the results showed that there were no obvious differences in the two sets of parameters. So a bio-supramolecular complex was formed after the reaction of amaranth and HSA, which induced the decrease of the free concentration of amaranth in the reaction solution, and the decrease of the reductive peak current of amaranth. The binding constant and binding ratio were calculated as 8.34 x 10(11) and 1:2, respectively. This new electrochemical method was further applied to the determination of healthy human serum samples and the results were in good agreement with the traditional Coomassic brilliant blue G-250 spectrophotometric method. (c) 2005 Elsevier B.V. All rights reserved.