A simple, sensitive, and selective method has been developed for determination of cysteine (Cys) or carbocysteine (carboCys) in pharmaceutical products, shampoos and a mixture of amino acids. The results showed the reaction between p-benzoquinone (PBQ) and Cys occurs through the sulfhydryl group. Previous derivatization or extraction is not necessary before the assay is carried out. The method is based on the fact that the product of reaction between PBQ and Cys absorbs at 352 and 500 nm or PBQ and carboCys absorbs at 500 nm. Beer's law is followed in the range 0-40 mug/ml for Cys and 0-150 mug/ml for carboCys. The product of reaction PBQ-Cys is stable for 2 h with absorption bands at 352 and 500 nm. In the presence of amino acids, PBQ is highly selective to Cys. Several substances such as amino acids, urea, salts, and dipeptide did not interfere with the proposed method. A recovery of about 100% is observed for both Cys and carboCys, when the method is applied to determine Cys in a mixture of amino acids resembling blood plasma, shampoo, and pill food as well as carboCys in pharmaceutical products.