A simple, sensitive and selective chemiluminescence (CL) method was developed for the determination of cysteine. This method is based on that the weak CL of cysteine oxidized with cerium (IV) can be greatly enhanced by quinine. The calibration curve was linear over the range 3.5 x 10(-9)-3.5 x 10(-6) M with a detection limit of 2.5 x 10(-9) M (S/N=3). The RSD was found to be 8.4% by 10 replicate determinations of 2.9 x 10(-8) M cysteine. Due to high sensitivity, the proposed method can be used directly to determine the total concentration of cysteine in human serum through simply diluting the sample for a thousand fold. The obtained result was in agreement with that given by amino acid autoanalyzer. The present method does not require any separation, showing a simpler analytical characteristic. The mechanism of the CL reaction was also discussed.