The hydrophobic polypeptide, polyproline, which has different chain lengths, was connected to the C-terminus of human lysozyme by recombinant DNA techniques. The hydrophobicity of human lysozyme increased with the increasing length of the polyproline chain. No significant perturbation of the secondary structure of lysozyme by incorporation of the polyproline chain was observed by circular dichroism spectroscopy. Although the bactericidal activity of wild-type human lysozyme is limited to Gram-positive bacteria (M. luteus and B. subtilis), the mutant lysozymes showed bactericidal activity to Gram-negative bacteria (E. coli and P. aeruginosa), and the activity increased with the increasing hydrophobicity of the mutant enzyme. Experiments with E. coli phospholipid liposomes revealed that the mutant lysozymes enhanced the release of the liposome contents, and this release increased with the increase of hydrophobicity. The increased hydrophobicity of the mutant enzyme may induce the interaction of lysozyme with the outer membrane E. coli and subsequent penetration into the inner membrane, resulting in the increase of bactericidal activity.