A paper chromatographic technique for the detection and identification of substances susceptible to darkening with polyphenoloxidase is described. Color formation occurs with the susceptible substances on the paper after it is sprayed with a polyphenoloxidase enzyme from apples. The principal browning substrate on chromatograms of the skin extract from Grimes Golden and Golden Delicious apple and Bartlett pear was l-epicatechin. Bartlett pear contained also smaller amounts of d-catechin, another browning substrate.