The role of cellular redox status in both cytotoxic activity and NF‐κ B activation in natural killer (NK) cells was investigated. The results indicate that stimulation of NK cells, either freshly isolated from peripheral blood lymphocytes (PBL) or long‐term cultured NK clones, with specific cell targets results in an increased binding activity of NF‐κB and AP‐1 transcription factors measured by gel retardation. Pretreatment of NK cells with the antioxidant pyrrolidine dithiocarbarmate (PDTC) leads to the inhibition of NF‐κB activation but the AP‐1 binding to DNA was superinduced. The inhibition of NF‐κB by PDTC paralleled with an inhibition of spontaneous cytotoxicity mediated by NK cells. Moreover, the inhibitors of serine proteases, N‐α‐tosyl‐l‐lysine chloromethyl ketone and N‐α‐tosyl‐l‐phenylalanine chloromethyl ketone, also blocked the cytolytic activity of NK cells against the sensitive target K562. In contrast, NK activity was not affected by pretreatment of the effector cells with the proteasome inhibitor N‐acetyl‐leu‐leu‐norleucinal which selectively inhibits NF‐κB activation. Altogether, these results support the hypothesis that the activation of NK cells involved transcriptional and post‐transcriptional events, and that reactive intermediates may play an important role in the molecular processes related with the generation of a cytotoxic response by NK cells.