Intracellular content of hydrogen peroxide and of the product of lipid peroxidation malonic dialdehyde as well as activity of antioxidant enzymes catalase, ascorbate peroxidase, and superoxide dismutase were studied in cells of morphogenic and derived from them non-morphogenic calluses of tatar buckwheat Fagopyrum tataricum L. Non-morphogenic calluses were characterized by significantly higher content of hydrogen peroxide and malonic dialdehyde, low catalase activity, and high activity of superoxide dismutase compared to morphogenic cultures. The results may indicate that cells of non-morphogenic calluses are in the state of continuous oxidative stress. Nevertheless, proliferative activity of non-morphogenic cultures and the biomass increase significantly exceeded these parameters in morphogenic calluses. An analogy is drawn between animal cancer cells and non-morphogenic plant calluses.