Anoxia followed by reoxygenation causes extensive damage to cellular components through generation of reactive oxygen intermediates. We examined cellular responses to oxidative stress after anoxia in cultured soybean or human fibroblast cells. Anoxia pretreatment protected soybean but not fibroblasts against H(2)O(2) concentrations that induced programmed cell death in normoxic cells. H(2)O(2) removal in anoxia-pretreated soybean cultures was faster. Protection was associated with increased action of alternative oxidase (AOX) and peroxidases. AOX inhibitors abolished the protective effect, while induction of AOX protected normoxic cells against H(2)O(2). We propose that during anoxia, plant cells can prepare for reoxygenation injury by up-regulating their antioxidant capacity, and that AOX is involved in this process.