Salts at high concentrations may cause oxidative damage to plant cells since many studies indicated the involvement of reactive oxygen species in salt-stress response. Recently, we have demonstrated that treatment of tobacco (Nicotiana tabacum) cell suspension culture with various salts result in an immediate burst of superoxide production via activation of NADPH oxidase by ions of alkali metals (Li+, Na+, K+), alkali earth metals (Mg2+, Ca2+) or lanthanides (La3+, Gd3+). In this study, we tested the effect of extracellular supplementation of Zn2+ and Mn2+ on the cation-induced oxidative burst in tobacco cell suspension culture, measured with a superoxide-specific Cypridina luciferin-derived chemiluminescent reagent. Extracellular supplementation of Zn2+ and Mn2+ inhibited the generation of superoxide in response to addition of salts. Although both Zn2+ and Mn2+ inhibited the salt-induced generation of superoxide, the modes of inhibition by those ions seemed to be different since Mn2+ simply inhibited total production of superoxide while Zn2+ inhibited the early phase of superoxide production and induced the slow release of superoxide. Roles of Mn2+ and Zn2+ in protection of plant cells from salt stress, as an effective superoxide scavenger and an effective inhibitor of plasma membrane-bound NADPH oxidase, respectively, are discussed.