The action of trypsin on the following amino acid derivatives has been investigated: the ethyl esters of ϵ-N-mono-, ϵ-N-di-, and ϵ-N-tri-methyl-L-lysine; the ethyl esters of the homologues of lysine and arginine; the methyl ester and amide of the α-N-benzoyl-DL-homolysine; the methyl esters and amides of the α-N-benzoyl derivatives of ϵ-N-di- and ϵ-N-tri-methyllysine; and poly ϵ-N-methyllysine. Derivatives ofL-ornithine,DL-2,8-diaminooctanoic acid, ϵ-N-dimethyl-, ϵ-N-trimethyl-, and ϵ-N-formyl-L-lysine were not substrates of trypsin. ϵ-N-Dimethyl-L-lysine derivatives did not inhibit the action of trypsin on a specific substrate.DL-Homolysine derivatives were hydrolyzed withkcat's one to two orders of magnitude lower than those of lysine derivatives, but theirKm's were only 1.5–3 times higher. ϵ-N-Methyl-L-lysine derivatives were hydrolyzed at rates similar to those forDL-homolysine derivatives, and hadKm's 25–115 times those of lysine derivatives. Plots ofKmandkcat/Kmversus side-chain length of the substrate for the ethyl esters of all the homologues of lysine and arginine indicated a correlation between these kinetic constants and side-chain length, and that the best substrate would have a side-chain length between those of lysine and arginine. Poly-ϵ-N-methyl-L-lysine was degraded to small peptides by trypsin.