The mechanism of the electrochemical reaction of lysozyme (Ly(SS),) at the mercury hanging drop electrode (HMDE) was investigated in Na2HPO4 + citric acid (pH 5). A pair of broader cathodic and anodic peaks appears at the same potential. The peak potential is -0.47 V vs. SCE. When a fresh mercury drop of the HMDE is pre-equilibrated with the stirred solution for a certain time at a potential about 0 V vs. SCE, only a sharp narrow cathodic peak is observed at the same potential as the peak without the pre-equilibrium and there is no peak corresponding to its anodic process. In the two eases, the mechanism of the electrochemical reaction is different. The reaction scheme without the pre-equilibrium between the mercury electrode and the solution containing lysozyme can by written as follows : Ly(SS)(4)(soln) reversible arrow Ly(SS)(4)(ads); Ly(SS)(4)(ads) + 2H(+) + 2e(-) = Ly(SS)(3)(SH)(2)(ads). In the case with the pre-equilibrium the reaction mechanism may be : Ly(SS)(4)(soln) reversible arrow Ly(SS)(4)(ads); Ly(SS)(4)(ads) + Hg reversible arrow Ly(SS)(3)(SS2-)Hg2+ (ads); Ly(SS)(3)(SS2-)Hg2+ (ads) + 2H(+) + 2e(-) --> Ly(SS)(3)(SH)(2)(ads) + Hg; Ly(SS)(3)(SH)(2)(ads) - 2H(+) - 2e(-) reversible arrow Ly(SS)(4)(ads). The two kinds of reaction proceed on the disulfide bond at the position of 6-127, because it is at the edge of the dimensional structure of the molecule of lysozyme.