The membrane immunoglobulin (MIg) of B lymphocytes is thought to have an important role in antigen recognition and cellular activation. In common with many membrane glycoproteins, MIg moves extensively in the lipid bilayer, and after binding of specific antisera displays lateral mobility with patch and cap formation. This phenomenon appears to involve the cytoskeleton, particularly the actin that is present in the cell membrane of B lymphocytes and aggregates beneath capped immunoglobulin. Recently, it has been reported that the isolation of MIg results in co-purification of actin and an unknown protein of molecular weight (MW) approximately 56,000 (refs 7, 8). We now demonstrate that this component displays physicochemical and immunological properties indistinguishable from those of Gc (group-specific component). In addition, evidence is presented which suggests that this vitamin D3-binding protein is involved in the linkage between MIg and actin, and may therefore be important in signal transduction.