The adaptive and cross-protection responses to oxidants were investigated in Bacillus sp. F26. The cells were treated with sublethal concentrations of either H2O2 or menadione (a superoxide-generating agent) to induce an adaptive response. The results showed that the cells treated with menadione exhibited cross-protection against, but in another case, those cells treated with H2O2 did not show significant resistance to menadione. It suggests that Bacillus sp. F26 possesses two separate adaptive responses that respond to the two different kinds of oxidants. The adaptability is regarded as that which is accompanied by the inductions of some antioxidant enzymes. It was found that catalase (CAT) production was increased about 1.6-fold after treatment with 600 mu M H2O2, whereas the presence of 50 mu M menadione induced CAT, superoxide dismutase (SOD), glucose-6-phosphate dehydrogenase (G6PD), and glutathione reductase (GR) by 2-, 2-, 2-, and 1.6-fold, respectively. The results can be used to explain why menadione-treated cells have higher adaptability to lethal concentrations of oxidants than that of those H2O2-treated. In addition, it was found that growing Bacillus sp. F26 in high-salinity media causes it to become more resistant to H2O2 and menadione stress, which may be partially due to the induction of CAT and SOD production under high NaCl concentration.