Phagocytosis of a variety of inert and metabolically active particulates plays an essential role in metazoan nutrition, metabolic economy, and defense against viral, bacterial, and protozoan pathogens . With the development ofthe lysosome concept (1-3), the processes governing the intracellular digestion of ingested particles have been clarified. In contrast, the mechanisms which regulate the ingestion phase of phagocytosis remain poorly understood. The recognition of contractile proteins in the cytoplasm of a variety of nonmuscle cells (4-7), including macrophages (4, 5, 7), has resolved one mechanism whereby chemical energy released during phagocytosis may be coupled to mechanical processes requisite for membrane movement and particle interiorization. However, no information is available which suggests how a phagocyte directs this system of contractile elements in order to engulf specific particles which are attached to its plasma membrane . That the process is indeed specific and limited to the segments of plasma membrane to which the particles to be ingested are bound has been carefully documented (8,9).The fastidious appetite of phagocytes is also well known. These cells avidly ingest some particles with which they come into contact while they totally ignore others (8, 9) . The presence or absence of opsonins (complement and immunoglobulins) on the surface of a particle is widely recognized as the most important factor governing the fate ofthe particle . Particles coated with ligands such as IgG and/or complement are avidly ingested by phagocytes while the same particles lacking such ligands are neither bound to, nor ingested by, phagocytic cells.The experiments reported in this paper were performed to examine the mechanisms which govern the vectorial and discriminatory qualities of the phagocytic process and to define the factors responsible for the segmental nature