Cell-restricted membrane antigens have made it possible to map the distribution of mature macrophages in many murine tissues. Monoclonal antibodies (mAbs) have been used to define the appearance of macrophages dusing foetal and postnatal development, to establish the anatomic relationships between macrophages and other cells in the normal and diseased adult, and to investigate cellular modulation and heterogeneity within different microenvironments. Current studies have illustrated the complex differentiation pathway of mononuclear phagocytes in vivo and have raised questions concerning the mechanisms that determine monocyte entry, migration and fate within tissues. Macrophages constitute a major, widely dispersed system of cells that regulate homeostasis in the normal host and respond to tissue injury by contributing essential functions during inflammation and repair. In this review we consider several membrane marker antigens which have proved useful in studying the life history and biologic properties of macrophages, and relate immunochemical studies on antigen expression to lineage analysis and macrophage differentiation in vivo. We restrict our discussion to the mouse, in which it is possible to manipulate the system in its entirety. Where known, properties of macrophages in other species are broadly similar.