The effect of B toxicity on antioxidant responses of soybean (Glycine max) cv. Athow was investigated by growing plants for 43 days at 0.2 (control), 2 and 12 mg B kg(-1). At the end of the treatment period, shoot growth, lipid peroxidation level, the activities of antioxidant enzymes including superoxide dismutase (SOD), peroxidase (POX), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR), and their isoenzymes in leaves were measured. Boron concentration in leaves was significantly increased by the increasing levels of B treatment from 43 to 522 mg kg(-1), and shoot dry matter was depressed at 12 mg B kg(-1). Significant increases in SOD, CAT, and APX activities were determined in leaves under 12 mg B kg(-1); however, GR activities were decreased while POX activity was unchanged. Increased enzymic antioxidant activity arose from a combination of newly formed isoenzymes and activation of existing isoenzymes. By contrast, SOD and GR activities were decreased by 2 mg B kg(-1) concentration as compared to the control groups while POX activity was increased and the activity of CAT did not change. Malondialdehyde content increased under 2 mg B kg(-1) but decreased under 12 mg B kg(-1). These results suggest that higher antioxidant activity observed under 12 than at 2 mg B kg(-1) provided higher free radical-scavenging capacity, and thus a lower level of lipid peroxidation in Athow. While the induction of increased antioxidant activity was related to internal boron levels, the signaling and coordination of responses remain unclear.