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The ultrastructural distribution of membrane-bound immunoglobulin (Ig) on mouse spleen lymphocytes has been studied by labeling the surface Ig with anti-mouse Ig antibody conjugated t o ferritin (FT). The distribution is temperature-dependent. At 0 OC -4 OC FTzppears p t r i b u t e d in small clusters over the entire surface, whereas at 2 0 C -22 C, in the great majority of the labeled cells, it is concentrated over one pole. This pole corresponds t o that part of the cell which contains the G o l d complex and most of the cellular organelles, and which forms the uropod in moving lymphocytes. This polar redistribution is accompanied by endocytosis of part of the labeled membrane.The redistribution of label occurring at room temperature indicates that labeled and unlabeled components of the plasma membrane can segregate from each other and suggests that plasma membrane is fluid at room temperature. The diffuse distribution found at 0 OC -4 OC is probably close (although not identical) t o the distribution in vivo before labeling, while the polar distribution at higher temperature is probably induced by the cross-linking of the surface Ig molecules by the antibody-FT conjugate and by a metabolically-dependent flow of the aggregates towards the "tail" of the lymphocyte. The mechanism involved is considered to be the same mechanism responsible for membrane flow during cell movement. A detailed model of the structure and rheological properties of the lymphocyte membrane is proposed, which can explain the present results and those of others concerning the behavior of surface components of lymphocytes and other cells.