The eutrophication of the Sulejów reservoir dam in Poland is connected with the problem of toxicity of cyanobacterial blooming (blue-green algal blooming). The main species responsible for hepatotoxic ”algal bloom” formation is Microcystis aeruginosa. The aim of this study is to evaluate the influence of the toxic cyanobacterial extract containing microcystins on the growth and morphology of a water plant (Spirodela oligorrhiza) and animal cells (rat hepatocytes). A higher concentration of cyanobacterial extract (MC-LR = 343 µg/dm3) reduced the number of fronds by about 50% in comparison with the control. The extract affected the reduction mass of fronds and the concentration of chlorophyll. The activity of the constitutive acid phosphatase decreased. The first morphological changes in rat hepatocytes typical of apoptosis were observed after 30 minutes of incubation with the cyanobacterial extract. The hepatocytes underwent cell membrane blebbing (MC-LR = 100 µg/dm3). The next 30 minutes of incubation caused an increase in the percentage of deforming cells of more than 50% (MC-LR = 100 µg/dm3). High chromatin condensation and apoptotic bodes were observed in 90% of cells after 120 minutes (MC-LR . 500 µg/dm3). The results of studies confirm the high toxic and cytotoxic effect of blue-green algal blooming from Sulejów reservoir on both plants and animals.