Intoxications with microcystins have lead to death of humans and animals such as livestock or fish. In order to determine the localization of microcystin in trout, an antibody was developed against MC-LR and tested for suitability of toxin detection in liver and other tissues of gavaged trout and in isolated primary trout hepatocytes. Polyclonal as well as monoclonal antibodies against microcystin-LR were produced and used for microcystin detection in tissue homogenates or in tissue sections. Microcystin was probed in tissue, cell and subcellular homogenates after in vivo and in vitro exposure to the toxin by immunoblotting and immunocytochemistry. In order to identify putative protein adducts, Western blots were also stained with anti-human protein phosphatase 2A (PP2A). Organ pathology was described in hematoxylin & eosin stains. The results show that the antibodies can detect microcystin-LR in trout organs and hepatocyte homogenates. Several microcystin-protein-adducts with a molecular weight of approximately 35 kD could be characterized. One of these adducts was identified as PP2A. Immunoblotting of hepatocyte proteins showed that the activity was largely localized in the cytosol, while immunocytochemistry demonstrated cytosolic as well as nuclear staining. Immunohistochemistry revealed a time dependent discernible increase in staining intensity throughout the liver, concurring with the kinetics of hepatic PP-inhibition. Organ pathology showed hepatocyte necrosis as an early event with secondary apoptotic changes after 48 hours at the earliest. This study suggests that accumulation of MC and subsequently changes in cellular morphology, PP-inhibition and hepatocyte necrosis represent the primary events in microcystin induced hepatotoxicity, while apoptotic cell death may be only secondary.