Consideration of membrane receptors and their modulation has assumed an increasing importance in our current understanding of the immune system. Basic cell mechanisms for establishing or altering membrane organization have a particular relevance for immunobiology with respect to the membrane-dependent interactions between cells or between a cell and soluble ligands. While our understanding of the organization of the cell membrane has come about from analysis of many cells, but most notably the red cell, our knowledge of the re-organization of the membrane upon interaction of a surface protein with ligands has derived to a great extent from the studies made on lymphocytes. One of the most fascinating and interesting of membrane reorganization phenomena concerns the sequence of membrane and cytoplasmic changes that ensue when B lymphocytes are exposed to antibodies to surface Immunoglobulin (Ig). As first observed by Taylor et al. (1), the lymphocyte rapidly and selectively redistributes its ligand-complexed antigen receptors to one pole of the cell, forming a cap.