The purpose of this study was to entrap poly(L-lysine) (pLys) as a surface active functional (SAF) polymer in the surface of biodegradable poly(DL-lactide-co-glycolide) (PLGA) microparticles, This was performed by stabilizing a methylene chloride-in-water emulsion with PLGA dissolved in the dispersed phase and using modified pLys as an emulsifier. The microparticles form as the organic solvent evaporates and PLGA precipitates entrapping the pLys. Modification of surface activity of pLys was accomplished by adjustment of ionization and secondary structure. PLys with greater than or equal to 68% degree of dissociation and greater than or equal to 50% a-helix content stabilized the PLGA oil-in-water emulsion and promoted surface entrapment of the polypeptide. The elevated a-helix content of ionization-modified pLys was essential for the stabilizing effect probably by formation of a strong interfacial film. The xi -potential measurement of PLGA microparticles prepared with 85% dissociated pLys showed a strongly positive charge, compared with negative values on control microparticles prepared with an un-ionized emulsifier, indicating the presence of the pLys on the microparticle surface. PLys was retained to > 60% on the microparticles following exposure to physiological conditions for 3 weeks.