Cytotoxic effects of crude extracts and fractions of the purification steps towards Microcystin-LR (MCYST-LR) were investigated in vitro. Cytoxicity was evaluated by measure of lactate dehydrogenase liberation of Chang liver cells and by hemolysis. Crude extracts of strain PCC 7806 damaged the cells within a few minutes. In contrast, MCYST-LR did not show any detectable cytotoxic effects. The cytotoxic activity could be related to a heat-labile substance with a molecular weight of about 30,000 Da.