This reaction is equal to the well known nitroprusside reaction of cysteine except that zinc chloride is used and that is more stable and delicate. Method: To a small amount of cysteine solution under examination in a test tube and a few drops of zinc chloride solution faintly acidified with hydrochloric acid and a freshly prepared aqueous solution of sodium nitroprusside. Neutralize with ammonium hydroxide or caustic alkali solution until the precipitation of zinc hydroxide occurs. A ruby red color resulted, which soon turns yellow in the case of the ordinary method of the reaction, remains for a few hours or days in this case with zinc salt. As the reaction is based on the adsorption of the colorine matter and cysteine into the white precipitate of the hydroxide, salts of some other metals such as magnesium or alminium, which produce white precipitate of hydroxide by the neutralization of their solutions, produce an effect somewhat similar to that of zinc salt, but the latter seems to be the best, for use, of all those examined.