Bioproductivity, biosynthesis of carotenoides, catalase activity and lipid peroxidation (LPO) of control cells as well as cells exposed to chronic UV-B radiation have been studied in the presence of the generators of active oxygen forms – methylene blue and herbicide norflurazon. Bioproductivity of the cells decreased when treated with norflurazon (NF) and methylene blue under intensive cultivation of the algae at chronic UV-B radiation (4.5 *103 ergs/mm2 per an hour). NF was found to suppress, while methylene blue and chronic UV-B radiation stimulate biosynthesis of carotenoids. Acting individually these stressors increased intracellular catalase activity, whereas joint action of the stressors caused synergetic suppression of catalase activity in Dunaliella cells. NF suppressed, but methylene blue and joint action of suppressors stimulated the process of LPO.