CHEMICAL methods using cyanogen bromide and aromatic amines1–3 (Konig reaction) and an acid-indicator4 for the detection of pyridine carboxylic acids on paper chromatograms have been reported. But the toxic character of cyanogen bromide causes difficulty; and it has been observed that good spots with acid-indicator are often not produced, particularly when the acid concentration is very low. A very convenient method has been found for detecting these acids on the filter paper by spraying with a solution containing cupric ions followed by benzidine; also their efficient separation from the mixture has been achieved.