Fc receptors for immunoglobulins are found on many immune cells and trigger essential functions of the immune defence system. With the exception of the high-affinity receptor for immunoglobulin E (Fc epsilon RI), these receptors were thought to consist of single polypeptides. Fc epsilon RI is a tetrameric complex of one alpha-subunit, one beta-subunit and two gamma-subunits. Here we report the cloning of a polypeptide identical to the gamma-chains of Fc epsilon RI, from mouse macrophages that do not express this receptor. Biosynthetic labelling and gene transfer together show that these gamma-chains associate with one of the macrophage receptors (Fc gamma RIIa). The human homologue, Fc gamma RIII (CD16), from natural killer cells is also expected to associate with gamma-chains. It is possible that these gamma-chains and the homologous zeta-chains of the T-cell antigen receptor belong to a new family of related proteins which share a common role in the signal transducing pathway.