Effect of an algicidal product from Oscillatoria late-virens and of the herbicide 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU) on growth, photosynthesis, and mouse toxicity in Microcystis PCC 7820 was examined. Their lethal concentrations abolished photosystem (PS)-II reactions and eventually bleached and detoxified the cyanobacterium. Although loss of protein, chlorophyll, and toxicity were also induced by sublethal antibiotic doses, photosynthetic activities remained unchanged and developed antibiotic tolerance. These effects could be duplicated in natural conditions, implying utility of the natural algicide in control of toxic cyanobacteria.