We have previously shown that myeloma-derived or serum-derived immunoglobulin could facilitate in vitro alloantigen recognition by immunocompetent thymus cells ('subset-I thymocytes'). The present report demonstrates the isolation of surface immunoglobulins from radiolabeled splenic B lymphocytes. Solubilized radiolabeled preparations of surface immunoglobulin are shown to bind in vitro to lymphoid cells, especially to immunocompetent thymus cells. Immunocompetent thymus cells (subset-I thymocytes preferentially bind more IgG than IgM. Papain digests of IgG are also bound by subset-I thymocytes; preferential binding of the Fc piece is observed. Although the binding of IgG or its pieces per se does not activate lymphoid cells in vitro a role for the binding of IgG during alloantigen recognition in vitro between immunocompetent thymus cells is demonstrated.