From the reaction between l-lysine and formaldehyde, Nepsilon-formyl-L-lysine was isolated by means of ion-exchange column chromatography. The identification of Nepsilon-formyl-L-lysine was carried out by ion-exchange overpressured-layer chromatography (OPLC) and 1H NMR and 13C NMR spectroscopies. The m.p. and mixed m.p. values, the retention characteristics and the chemical shifts of the isolated product were identical with those of an authentic sample of Nepsilon-formyl-L-lysine.