Callus of Alternanthera philoxeroides was cultured by tissue culture technique, which was used as experimental material.This paper studied the toxic effects of Pb²⁺ gradient concentrations (0, 0.2, 0.4, 0.8 and 1.6 mmol/L) on the contents of photosynthetic pigments and soluble protein, the level of active oxygen, activities of antioxidase(GAT, SOD and POD) and changes of ultrastructure in callus of Alternanthera philoxeroides.Tlie results indicated that, with the increasing of Pb²⁺ concentration, the contents of photosynthetic pigments and soluble protein decreased gradually.The contents of O₂^- and H₂O₂ increased gradually, while the content of MDA increased at first and then decreased.TTie changes of SOD and POD activities were the same as MDA, but CAT activity decreased gradually.Transmission electron microscopy observation showed that the chloroplast, mitochondria and nucleus suffered serious damages under Pb²⁺ stress, and the concentration of 0.8-1.6mmol/L generated the fateful poisons to ultrastructure.It was concluded that Pb²⁺ stress induced not only destruction of physiological function and structural foundation of Alternanthera philoxeroides callus, but also the disorder of physiological metabolism, which led to the death of the callus at last.