An interaction between cysteine and cobalt(0) was investigated at a Nafion(R)/cobalt(II) modified electrode. In the presence of cysteine, cobalt(II) was reduced at lower overvoltage compared with that in the absence of cysteine. An oxidation peak of cobalt(0) was observed at -0.25 V (Ag/AgCl). As the shift of the overvoltage depends on the concentration of cysteine, cysteine could be determined by the oxidation peak on the basis of the shift. The calibration curve of cysteine was linear from 4 X 10(-8) to 1 X 10(-6) M. The relative standard deviation at 5 x 10(-8) cysteine was 5.8% (n = 5). In addition, the amount of cobalt participating in the electrode reaction was estimated. As a result, it was found that 1/30 of the cobalt in the electrode took pert in the electrode reaction.