Natural killer (NK) cells express surface receptors for defined groups of HLA class I alleles. The specific interaction between these receptors and HLA class I molecules expressed on target cells results in inhibition of NK-mediated target cell lysis. In this report, we analyzed whether similar mechanisms were operating in cytolytic T lymphocytes (CTLs) capable of lysing NK-sensitive target cells. T cell clones were screened for their ability to lyse K562 target cells. The selected clones expressed either γδ or αβ TCR. The majority of these clones failed to lyse the HLA class I⁺ R8/15375 cell line; however, upon addition of the previously described A6–136 (IgM) or 6A4 F(ab')₂ anti-HLA class I mAbs, target cells were efficiently lysed. Lysis of autologous phytohemagglutinin blasts in the presence of anti-HLA class I mAbs occurred primarily with TCRγδ⁺ CTLs. Recognition of HLA class I molecules on target cells implies the expression of NK-related specific receptors in CTL clones. Indeed, phenotypic analysis of ≺300 CTL clones with NK-like activity revealed that 28% expressed p58 molecules (specific for HLA-C alleies) while 30% expressed CD94 molecules (specific for the Bw6 specificity). These receptor molecules were found to function as inhibitory receptors, as revealed by the effect of anti-p58 or anti-CD94 mAbs (of IgG isotype) on the lysis of the FcγR⁺ K562 target cells. In addition, while the 221 cell line was susceptible to lysis by CTL clones, 221 cells transfected with Cw3 were not lysed by GL183⁺ (p58) clones, thus providing direct evidence for the specific protection exerted by given HLA class I alleles. Finally, we show that anti-p58 or anti-CD94 mAbs inhibit the T cell activation induced by anti-TCR mAbs. This inhibitory activity has been detected both on the TCR-triggered target cell lysis and on cytokine tumor necrosis factor-α production.