We have recently derived from human fetal blood (25 wks) a series of cloned cell lines that were selected for their ability to kill the conventional natural killer (NK) target cell K562¹. It was found that a fraction of these clones express CD3 proteins but not the monomorphic Ti αβ determinant recognized by WT31 antibody¹. One interleukin-2-dependent CD3⁺ WT31⁻ clone, termed F6C7, was used for immunization of mice to generate monoclonal antibodies directed at a potentially novel recognition receptor. It was shown that F6C7 cells, which transcribe Ti β but not Ti α genes, surface-express a clonotypic structure, termed NKFi². Immunoprecipitations performed with anti-NKFi monoclonal antibody (mAb) indicated that the corresponding molecule is resolved in SDS-polyacrylamide gel electrophoresis (PAGE) as a single band of relative molecular mass ∼85,000 (M_r∼85K). After reduction, a major band was detected at 44K and a faint band was present at 41K². The present study was designed to characterize this structure. It was found that NKFi represents either two 44K disulphide-linked γ (TCR) chains, or possibly one γ chain associated to an additional undetected molecule, and that the 4IK material corresponds to a partially glycosylated fraction of the γ protein. Anti-NKFi mAb both induces a specific autocrine pro-liferative response and blocks cytotoxic function, demonstrating that γ chains serve as functional receptor structures on subpopulations of normal human lymphocytes.