Summary: Glycine homopeptides are concentrated by Escherichia coli and are completely hydrolysed by intracellular peptidases. Glycine peptides in which a peptide-bond nitrogen is methylated, i.e. glycylsarcosine and glycylglycylsarcosine, still use the peptide transport systems, but the substituted peptide bonds are not hydrolysed. Glycylsarcosine is thus nutritionally inactive, but it competes with other dipeptides for uptake. Only the N-terminal residue of glycylglycylsarcosine is utilized by a glycine auxotroph. The results reflect the different specificities of the peptide transport systems and the intracellular peptidases of E. coli.