Evidence exists that antigen is trapped in lymphoid follicles complexed with antibody (1–6) and that antigen-antibody complexes (Ag-Ab) prepared in vitro localize in these same areas (7). The localization of the complexes depends on the presence of the Fc of the antibody since complexes of the antigen with Fab fragment do not localize in the follicles (8). These studies consider that the binding of Ag-Ab complexes and Fc fragment occurs by an Fc receptor present on reticular cells. We have previously described the presence of a receptor for antibody Fc on the surface of mouse spleen lymphocytes (9–11). This receptor could bind isolated Fc fragments as well as the Fc of antibody in Ag-Ab complexes. The binding was independent of complement. In the present communication we present some evidence for the possible biologic role of this receptor during the primary response and during the interaction of rabbit anti-mouse thymocyte serum (ALS) with lymphocytes both in vitro and in vivo.