Encapsulation of proteins in poly(lactic-co-glycolic) acid (PLGA) microspheres by the water-in-oil-in-water (w/o/w) technique is very challenging because of the inherent physical instability of proteins. In particular, exposure of proteins to the first water-in-oil emulsion causes unwanted interface-induced protein inactivation and aggregation. We tested whether salts could afford stabilization of a model protein, hen egg-white lysozyme, against the detrimental events occurring at the w/o interface and subsequently upon w/o/w encapsulation. First, we investigated the effect of salts on the specific enzyme activity and generation of soluble precipitates and insoluble aggregates upon emulsification of an aqueous lysozyme solution with methylene chloride. It was found that lysozyme precipitation occurred upon emulsification. The amount of precipitate formed at salt concentrations between 10-100 m M was related to the position of the anion in the electroselectivity series (SO42- > SCN- > Cl- > H2PO4-) while high salt concentrations (1M) led to > 80% of lysozyme precipitation regardless of the salt. The precipitates consisted of buffer-soluble protein precipitates and water-insoluble noncovalent aggregates. Lysozyme precipitation, aggregation, and inactivation upon emulsification were largely prevented in the presence of 50 mM KH2PO4 while KSCN caused an increase in these detrimental events. Second, it was tested whether the improved structural integrity of lysozyme at the w/o interface would improve its stability upon w/o/w encapsulation in PLGA microspheres. Some conditions indeed led to improved stability, particularly codissolving lysozyme with 50 mM KH2PO4 reduced loss in the specific activity and aggregation. In conclusion, the type and concentration of salts is a critical parameter when encapsulating protein in PLGA microspheres. (C) 2003 Wiley Periodicals, Inc.