Laboratory experiments targeting the effect of humic acid (HA) on the cell lysis of cyanobacterium Anabaena circinalis have been performed. Light irradiation was found to be an important factor for the cell lysis phenomenon, whereas intracellular hydrogen peroxide (H2O2) might be a chemical factor for the process. An exogenous H2O2 concentration of 1.0 mg l(-1) was determined as the threshold for cell survival. Our results indicated that HA or its possible product(s) of photochemical reaction can induce damage to intracellular catalase under artificial illumination, which leads intracellular H2O2 to be accumulated to an abnormally high concentration, eventually resulting in cell death. Moreover, H2O2 released into the culture from dead cells can damage other cells, which in turn brings about the population extinction.