Extracts from blue‐green algal blooms (Microcystis aeruginosaandOscillatoria agard‐hii) from different lakes in southeastern Norway were tested for toxicity toward freshly prepared rat hepatocytes. The toxicity effects were scored by means of morphological studies of the cells and by measuring leakage of the enzyme lactate dehy‐drogenase (LDH) from the cells. The results with the hepatocytes correspond well with results from the traditional mouse bioassay, concerning both ability to distinguish between toxic and nontoxic samples and estimation of relative toxicity. Morphological changes due to toxic effects on the plasma membrane appeared earlier than leakage of enzyme from damaged cells. The results indicate that the hepato‐cyte‐toxicity assay system might be well suited for screening purposes concerning water contamination by blue‐green algae.