Abstract A rapid automated method for the determination of serum methionine is presented for use with the Technicon Auto Analyzer system. It is based on the decolorisation of halide platinates by organic sulphides, including methionine. The 5–95% reference range from 249 sera from hospital patients (excluding those with liver and renal diseases and diabetes) was 20–60 μmol/1, mean 35 μmol/l. The S.D. of the method for 40 duplicate samples was 2.2 jumol/l. Recoveries ranged from 101–105%. This measurement facilitates the clinical evaluation of liver disease, especially acute liver failure, kidney disease and diabetes mellitus.