Microcystin-LR (MC-LR) is the most commonly encountered of the toxic cyclic peptide hepatotoxins occurring in China. It is a model compound for toxicological studies. In this study, the toxicity of MC-LR (50 and 500 micrograms/L) on isolated carp hepatocytes was determined and the ultrastructural alterations of cells induced by MC-LR were observed. The alterations noted when hepatocytes were exposed to 50 micrograms/L MC-LR were blebbing of cell membrane, shrinking and deformation of nuclei, vesiculation and transformation into concentric membrane whorls of RER, and swelling and rearrangement of the cytoskeleton. However, when the cells were exposed to 500 micrograms/L MC-LR, broken cell membranes, nuclei and cytoskeleton could be observed. These ultrastructural changes paralleled the pathological events, which lead to apoptosis or necrosis of hepatocytes. These results suggest that disruption of the cytoskeletal structures could account for the blebbing of cell membrane and apoptosis induced by MC-LR.