STIMULATION of T cells by antigen activates many signalling pathways1. The capacity for this range of biochemical responses may reside in the complex structure of the seven-chain T-cell antigen receptor (TCR)2. In addition to the complexity shared by all TCRs, coexpression of zeta (ζ) and the distinct but related eta (η) chain creates structural diversity among the TCR complexes expressed on a given cell3,4. In most murine T cells that we have studied, about 90% of the heptameric receptor complexes contain a ζζ disulphide homodimer, whereas 10% contain a ζη disulphide heterodimer5. Recent studies suggest that ζ has a critical role in allowing antigen to activate the cell, whereas η expression has been correlated with the capacity for antigen-induced phos-phoinositide turnover6,7. A third ζ-related protein, the gamma (γ) chain of the Fee and some Fcγ receptors, exists as a disulphide homodimer in those complexes8-10. The structural relatedness of ζ and γ is emphasized by the recent demonstration of ζζ in association with CD16 in TCR-negative natural killer cells11-13. Here we identify T cells lacking Fc receptors but coexpressing ζ, γ and η, document the formation of novel heterodimers between ζ and γ and between η and γ and show their association with the TCR. A greater range of homologous coupling structures than previously thought may be one way of achieving the variety of TCR-mediated (and possibly Fc receptor-mediated) biochemical responses and effector functions. © 1990 Nature Publishing Group.