We have previously reported that monochloramine (NH₂Cl), a neutrophil-derived oxidant, inhibited tumor necrosis factor α (TNFα)-induced expression of cell adhesion molecules and nuclear factor-κB (NF-κB) activation (Free Radical Research 36 (2002) 845–852). Here, we studied the mechanism how NH₂Cl inhibited TNFα-induced NF-κB activation, and compared the effects with taurine chloramine (Tau–NHCl). Pretreatment of Jurkat cells with NH₂Cl at 70 μM resulted in suppression of TNFα-induced IκB phosphorylation and degradation, and inhibited NF-κB activation. In addition, a slow-moving IκB band appeared on SDS-PAGE. By contrast, Tau–NHCl for up to 200 μM had no effects. Interestingly, NH₂Cl did not inhibit IκB kinase activation by TNFα. Protein phosphatase activity did not show apparent change. When recombinant IκB was oxidized by NH₂Cl in vitro and phosphorylated by TNFα-stimulated Jurkat cell lysate, its phosphorylation occurred less effectively than non-oxidized IκB. In addition, when NF-κB–IκB complex was immunoprecipitated from NH₂Cl-treated cells and phosphorylated in vitro by recombinant active IκB kinase, native IκB but not oxidized IκB was phosphorylated. Amino acid analysis of the in vitro oxidized IκB showed methionine oxidation to methionine sulfoxide. Although Tau–NHCl alone had little effects on TNFα-induced NF-κB activation, simultaneous presence of Tau–NHCl and ammonium ion significantly inhibited the NF-κB activation, probably through the conversion of Tau–NHCl to NH₂Cl. These results indicated that NH₂Cl inhibited TNFα-induced NF-κB activation through the oxidation of IκB, and that NH₂Cl is physiologically more relevant than Tau–NHCl in modifying NF-κB-mediated cellular responses.