A simple and rapid atomic absorption method for the determination of some amino acids containing a sulphur group is described. The studied compounds are cysteine, cystine and methionine. The method is based on the addition of an excess of mercury (II) chloride in phosphate buffer, pH 9, forming a white precipitate; the unreacted mercury is separated by centrifugation and the mercury ions in both precipitate and filtrate are determined by atomic absorption spectrophotometry at 253.6 nm. The concentration of the studied amino acids is then indirectly determined from a calibration curve for standard mercury (II) chloride solution.