A method was developed for estimation of concentrations of cystine based on the reaction of this oxidised amino acid with ascorbic acid and copper ion. The standard curve which was constructed was used as the basis for investigation of the effects which both cystine and glutathione disulphide, and some disulphide proteins, exerted on the amount of hydrogen peroxide formed in the reaction of ascorbic acid. The hydrogen peroxide product was lowered in the presence of the disulphide compounds. The disulphide compounds also lowered the amount of deoxyribose reacting material formed during the ascorbic acid oxidation. The reaction between ascorbic acid and copper and cystine was used to estimate the amount of the disulphide amino acid in acid digests of proteins.