We have recently derived a series of cloned cell lines¹ displaying natural killer (NK) cell-like activity from normal human fetal blood (25 weeks)². The lines were obtained after repeated stimulation of mononuclear cells with allogeneic Epstein–Barr virus (EBV)-transformed B lymphocytes and are interleukin-2 (IL-2) dependent. Initial characterization of the clones has been reported previously¹. Certain of these clones have been found to have unusual surface characteristics, namely, they are recognized by several well-defined anti-T3 antibodies, but do not react with WT31 (ref. 3), which is thought to recognise an invariant epitope of the human (Ti-αβ) structure^4,5. Transcription of the genes encoding the α- and β-chains of the T-cell receptor was assessed in two of these clones (F6A4 and F6C7). Ti-β genes were found to be expressed, whereas α messenger RNA was not detected in Northern blot analysis. These data strongly suggest that these cells do not produce a stoichiometric T3/Ti-αβ receptor complex. However, experiments performed with a monoclonal antibody (anti-NKFi) developed against F6C7 cells demonstrated the existence of a unique clonotypic structure [relative molecular mass (M_r) 85,000 (85K)] which is surface-associated with T3 proteins. Furthermore, both anti-T3 and anti-NKFi were found to block cytotoxic effector function. Together, the results support the view that T3 proteins are involved in non-major histocompatibility complex (MHC)-restricted cytotoxic reactions mediated by certain circulating fetal lymphocytes which are likely to use a clonotypic structure distinct from both the ‘first’ (αβ)⁴ and the putative ‘second’ (γδ)⁴ T-cell receptor to recognize their target. The present studies were designed to characterize this structure.