The use of polymeric microsphere systems (including polypeptides) as vehicles for delivering drugs by a variety of routes is considered with particular reference to parenteral administration. Microsphere formulation is discussed with emphasis on the role of surface properties and how these can determine the fate of administered particles. Studies on the use of adsorbed block copolymers that can alter processes of protein uptake onto particle surfaces (opsonization) as well as the interaction of particles with macrophages (cell-particle adhesion) are described. Data are presented which show that by the selection of the appropriate coating material it is possible to direct particles within the body to sites such as the lung, liver, bone marrow, or to retain materials within the systemic circulation.