Biologically important sulphydryl compounds, e.g. cysteine, were determined by treating the reductant thiol with copper(II)-neocuproin (Nc) reagent in ammonium acetate-buffered medium, followed by measuring the absorbance of the copper(I)-Nc complex at 450 nm. Cystine, if present in a mixture with cysteine, was reduced to the latter with zinc-hydrochloric acid prior to the colour reaction. Many common amino acids (not containing the SH group) and reducing sugars did not interfere, and colour development took 2 min. Ascorbic acid, glutathione and penicillamine gave similar colour reactions to cysteine. For cysteine, the molar absorptivity was 7.5 X 10(3) l mol-1 cm-1 with a relative standard deviation of 3%, rendering the method suitable for routine work with good precision. The method was successfully applied to the assay of cystine in soybean protein hydrolysate, and the results were compared to those obtained by liquid chromatography.