A spectrophotometric method for the determination of cysteine and cystine in peptides and proteins is described. The method is sensitive and simple and is based on the formation of a coloured complex with osmic acid. It is specific for —SH and/or for —S—S— or CS groups that can be hydrolysed by NaCN to —SH. Both cysteine and cystine could be determined directly in untreated proteins. Hydrolysis was not necessary. Methionine and other amino acids did not react. Depending on the cysteine content, as little as 0.01 mg of protein could be analysed. Carbohydrates, nucleic acids, alcohols, aldehydes, amines, surface-active reagents and common salts did not interfere. Free cysteine and/or cystine in the mixtures was determined at concentrations as low as 10^–5M.