Summary An improved method for the quantitative assay of complex amino acid mixtures has been described. Complete separation of amino acids in simple protein hydrolyzates has been achieved by one-dimensional paper chromatography using a number of alcohol solvents. Direct photometric scanning of these one-dimensional chromatograms subjected to a new modification of the ninhydrin reaction (or isatin reaction in the case of proline) is the basis of the assay. The coefficient of variation for a single determination is ±4.36%. The analysis of bovine serum albumin and of a synthetic mixture of amino acids has been reported.