Extended protein release from readily prepared, water-insoluble complexes with oppositely charged polyions is explored. Using hen egg-white lysozyme as a model, its sustained release from such complexes with a number of polyanions under physiological conditions has been demonstrated and rationalized. The rate of release varies orders of magnitude and is controlled by the nature of the polyanion (decreasing upon increase in its linear charge density, length, and hydrophobicity) and the complex particle size (the larger the particles, the slower the release). (C) 2003 Wiley Periodicals, Inc.