The voltammetric behaviour of cysteine was investigated in the presence of Cu(II) ions at a rotating glassy carbon electrode (RGCE). Two reduction peaks related to the successive reduction of the cysteine complexes of Cu(II) and Cu(I) ions were observed at -0.30 and -0.60 V vs. SCE respectively. These peaks were increased upon the addition of cysteine into a pH 9.20 borate buffer solution containing 0.1 M KNO(3) and 1.0 x 10(-5) M Cu(II) ions. The calibration curve was linear in the concentration range of 5.0 x 10(-10) - 2.0 x 10(-8) M for cysteine by utilising the peak current at -0.6 V vs. SCE. In the flow technique, 50 muL portions of off-line prepared mixtures of 1.0 x 10(-5) M Cu(II) and varying concentrations of cysteine were injected into the carrier solution of pH 9.20 borate buffer containing 0.1 M KNO(3) flowing through 25 cm transmission tubing (i.d. 0.75 mm) at a rate of 1.5 mL/min. The signals were monitored at -0.60 V vs. SCE. The calibration graph was found to be linear in the concentration range 1.0 x 10(-9) - 1.0 x 10(-7) M with a relative standard deviation of 1.0% (n = 5) for 5.0 x 10(-8) M cysteine. The detection limit was calculated as 5 x 10(-10) M for S/N = 3 ratio.