It has been found that the conversion of L-lysine (1) into its corresponding N epsilon-methylated derivatives, mainly N epsilon-mono-methyl-L-lysine (5; MML), but also N epsilon, N epsilon-dimethyl-L-lysine (DML) and N epsilon, N epsilon, N epsilon-trimethyl-L-lisine (TML) takes place by treatment with formaldehyde in spontaneous reaction. The identification of N epsilon-methylated lysines was carried out by different chromatographic and spectroscopic methods. This spontaneous N epsilon-methylation of L-lysine by formaldehyde may also play an important role in living organisms.