The heptapeptide toxins produced by the blue-green alga (cyanobacterium) Microcystis aeruginosa are selectively hepatotoxic in mammals. The characteristic post-mortem pathology of the liver is extensive lobular disruption due to sinusoidal breakdown, leakage of blood into the tissue and hepatocyte disintegration. Isolated hepatocytes incubated with toxin show severe structural deformity and surface blebbing. This paper demonstrates the effects of Microcystis toxins on the contraction and aggregation of actin microfilaments, and on the relocation and breakdown of cytokeratin intermediate filaments, in cultured hepatocytes. Earlier work did not show changes in the assembly/disassembly of actin; however, this paper demonstrates the change in cytokeratin from intermediate filaments to distributed granules in the cytoplasm of toxin-affected cells. Acrylamide gel electrophoresis of cytoskeletal fractions from hepatocytes did not show changes in total cytokeratins; however, marked changes in the immunogenicity of cytokeratins at 52 and 58 kDa were seen on toxin exposure of cells. Measurement of 32P-phosphorylation of proteins in toxin-affected cells incubated with [32P]orthophosphate showed a dramatic increase compared to control incubations. This is in agreement with research elsewhere describing phosphatase inhibition in vitro by Microcystis toxins. The data indicate that phosphorylated cytokeratin is a major component of cytoplasmic fraction phosphorylated protein after toxin exposure to hepatocytes. It is concluded that the mechanism of Microcystis toxicity to the hepatocyte is through cytoskeletal damage leading to loss of cell morphology, cell to cell adhesion and finally cellular necrosis. The underlying biochemical lesion is likely to be phosphatase inhibition causing hyperphosphorylation of a number of hepatocyte proteins, including those cytokeratins responsible for microfilament orientation and intermediate filament integrity.