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 (FcɛRI), these receptors were thought to consist of single polypeptides. FceRI is a tetrameric complex of one α-subunit, one β -subunit and two γ-subunits. Here we report the cloning of a polypeptide identical to the γ-chains of FcɛRI, from mouse macrophages that do not express this receptor. Biosynthetic labelling and gene transfer together show that these γ-chains associate with one of the macrophage receptors (FcγRIIa). The human homologue, FcγRIII (CD 16), from natural killer cells is also expected to associate with γ-chains. It is possible that these ε-chains and the homologous ζ-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.