Cyanobacterial blooms in water reservoirs have multiple adverse effects to the quality of water abstracted from such reservoirs. Intensive blooms generate large amounts of organic matter, increase water oxygen consumption, cause the occurrence of numerous pathogens and unpleasant smells and, above all, they produce cyanobacterial hepatotixins. Their high toxicity and cancer induction potential combined with a high stability pose extremely serious health problems. That is why the toxins should be removed efficiently during water treatment. We present results of a study on the treatment of water containing cyanobacterial hepatotixins from the Zalew Sulejowski.