The clearance of liposomes from the circulation is mediated largely by the cells of the reticuloendothelial system. These cells recognize liposome-bound immunoproteins, particularly immunoglobulins and complement. This review addresses the state of knowledge of the plasma proteins that can initiate complement activation and antibody binding and the known interactions among these proteins and liposomes. Evidence for the involvement of immunoproteins and other molecules in the in vivo survival of liposomes is also reviewed.