Publisher Summary The choice of a method for the determination of cystine–cysteine in various materials depends largely on the nature of the material to be analyzed and the relative specificity of the method. To date, the most specific method for cystine-cysteine is based on the use of sodium-l,2-naphthoquinone-4-sulfonate. Various interfering substances were reported from time to time, but these either are not found in proteins, or are largely removed during hydrolysis, or their effect is minimized or abolished by the proper use of the procedure. The method is applicable to tricarboxylic acid cycle filtrates of tissue homogenates. Amounts of about 75γ of cystine in the volume of liquid used for analysis in the photoelectric colorimeter appear to be minimal for accurate and reproducible results. For determination of methionine two procedures are described. In one, iodine, which is bound by methionine in 1:1 molar ratio, is determined either titrimetrically or spectrophotometrically. In the other, methionine reacts with nitroprusside in strong acid medium, and the resulting color is measured at 540 mg. The iodimetric procedure is independent of methionine peptides, but not of S-substituted homocysteines or homocysteine thioacetals, homocystine, or homocysteine. The colorimetric procedure responds to methionine peptides and S-substituted homocysteines, but not to homocystine, homocysteine, homocysteine thioacetals, cystathionine, or homolanthionine.