Abstract: The plant hormone abscisic acid (ABA), as a stress signal, plays important roles in the regulation of plant responses to environmental stresses. The relationship among abscisic acid (ABA) accumulation, the generation of Reactive Oxygen Species (ROS) and the activities of several antioxidant enzymes such as Catalase (CAT), Ascorbate Peroxidase (APX) and Glutathione Reductase (GR) was investigated in leaves of detached bean (Phaseolus vulgaris L.) plants exposed to 10,100 and 1000 μM ABA [Detached leaves of bean were cut into leaf segments and floated in solutions containing 0, 10, 100 and 1,000 μM ABA, at 25°C and then used for assay of antioxidant enzymes]. Treatment with 10 and 100 μM ABA significantly increased the levels of O2¯ and H2O2, followed by an increase in activities of CAT, APX and GR in leaves of bean seedlings. Treatment with 1,000 μM ABA led to a more abundant generation of O2¯and H2O2 .The activities of these antioxidative enzymes were reduced when compared with the treatment of 100 μM ABA. These results indicate that treatment with low concentrations of ABA (10 to 100 μM) induced an antioxidative defense response against oxidative damage, but a high concentration of ABA (1,000 μM) induced an excessive generation of ROS and led to an oxidative damage in plant cells.