Brief exposure to ozone, a potent cross-inducer of plant stress responses, leads within minutes to activation of an ERK-type MAP kinase (approximately 46 kDa) in tobacco. This activation process is calcium-dependent and can be blocked both by free radical quenchers and by a specific inhibitor of MEK-1 (MAPKK). Hydrogen peroxide and superoxide anion radicals can substitute for ozone as the activation stimulus, which does not appear to require salicylate as an intermediary. The properties of the ozone-induced MAPK suggest that it may be SIPK (salicylate-induced protein kinase), a tobacco MAPK that is activated by a variety of stress treatments. The ability of ozone to activate SIPK indicates that this protein kinase acts as a very early transducer of redox stress signals in plant cells.