Natural-killer (NK) cells are large granular lymphocytes involved in host defense against tumor cells and virally-infected cells. In addition to natural cytotoxicity, NK cells can effect antibody-dependent cytotoxicity mediated by CD16 (Fc gamma RIIIA alpha). It has recently been shown that CD16 is associated with disulfide-linked dimers composed of 2 homologous sub-units, zeta and gamma. These transducing molecules are also associated with other multimeric cell-surface receptors such as the T-cell antigen receptor (CD3:TCR) complex (zeta and gamma) and the high-affinity Fc receptor for IgE (Fc epsilon RI) expressed on basophils and mast cells (gamma). These results show that distinct cell-surface receptors utilize common transducing sub-units, and emphasize the homology between the CD16, Fc epsilon RI and CD3:TCR complexes. However within the lymphoid cells, the gamma-gamma homodimer is preferentially expressed in NK cells and cytotoxic T cells, suggesting that specific combinations of these transducing dimers might sub-serve distinct signal-transducing functions, and contribute to lymphocyte heterogeneity.