The effect of 14 metal cations and NH4+ on the response current of a polyaniline glucose oxidase electrode is investigated in this paper. The immobilized glucose oxidase is only activated by Mn2+ among investigated cations. Based on the experimental results, a mechanism of the activation kinetics of the immobilized glucose oxidase in the presence of Mn2+ is presented, that is E + M reversible arrow EM, EM + S reversible arrow EMS --> EM + P, and a rate equation from this model is given. The inhibition of glucose oxidase by CU2+ is reversible and non-competitive.