2-hydroxy 1-4 naphthaquinone reacts with phenyl hydrazine hydrochloride in the hot state at pH 6.3 to give a red colour, which after extraction in benzene has a maximum absorbance at 450-480 my. The colour reaction is specific for Lawsone has 1 ,ug/1-0 ml as visual Iimit of identification and provides the basis of a new spectrophotometric method for the determination of 2-hydroxy 1-4- naphthaquinone. The quantitative assessment of the tolerable amounts of different organic compounds which do not interfere with the determination is reported.