The aetiology of sudden deaths of cattle in the Kanton Graubünden has been elucidated by a multi-disciplinary approach. Certain small rivers and ponds located in the affected Alpine pastures in the areas of Misox, Rheinwald and Engadin provide favourable habitats for the growth of hepatotoxic cyanobacteria, particularly during long periods of hot and dry weather. As cattle frequently take up water from these sources, the toxins produced by cyanobacteria may lead to lethal poisonings that are typically associated with hemorrhagic liver necrosis. The latest reported case of cyanobacteria poisoning of Alpine cattle occurred during the summer of 1996. Further animal losses may be avoided by the implementation of appropriate pasture management schemes. To identify sites of potential haz- ards to human or animal health, we have developed a sensitive screening assay for the detection of cyanobacteria hepatotoxins in algae and water samples.