Oxidation of a highly conserved cysteine (Cys) residue located in the kinase activation loop of mitogen-activated protein kinase kinases (MAPKK) inactivates mammalian MKK6. This residue is conserved in the fission yeast  Schizosaccharomyces pombe  MAPKK Wis1, which belongs to the H 2 O 2 -responsive MAPK Sty1 pathway. Here, we show that H 2 O 2  reversibly inactivates Wis1 through this residue (C458)  in vitro  We found that C458 is oxidized  in vivo  and that serine replacement of this residue significantly enhances Wis1 activation upon addition of H 2 O 2  The allosteric MAPKK inhibitor INR119, which binds in a pocket next to the activation loop and C458, prevented the inhibition of Wis1 by H 2 O 2   in vitro  and significantly increased Wis1 activation by low levels of H 2 O 2   in vivo  We propose that oxidation of C458 inhibits Wis1 and that INR119 cancels out this inhibitory effect by binding close to this residue. Kinase inhibition through the oxidation of a conserved Cys residue in MKK6 (C196) is thus conserved in the  S. pombe  MAPKK Wis1.