AbstractPeptide‐containing block copolymers poly(N‐acetyliminoethylene)‐block‐poly(L‐phenylalanine)s(1)form water‐soluble large aggregates in water due to the hydrophobic and hydrogen‐bonding nature of the poly(L‐phenylalanine) block. The solution properties were studied by size exclusion chromatography, dynamic light scattering,1H NMR spectroscopy, and transmission electron microscopy. The results were compared with those of the block copolymers poly(N‐acetyliminoethylene)‐block‐poly(N‐benzoyliminoethylene)s(2)as analogous structures of poly(L‐phenylalanine). The peptide‐containing block copolymers1form aggregates in water even though only short segments of poly(phenylalanine) are provided. The hydrogen‐bonding character of the amide bond in the poly(phenylalanine) segment may induce strong interactions with each polymer chain in water. The interaction of Lipase P with the polymer aggregates in water was studied by means of size exclusion chromatography. The aggregates have a strong capability of incorporating Lipase P and largely increase the hydrolysis activity of Lipase P againstp‐nitrophenyl propionate compared with free Lipase. The guest‐binding ability of 8‐anilino‐1‐naphthalenesulfonic acid for