Harmful algal blooms (HABs) consisting of cyanobacterial species have been increasing in waterways in the United States and elsewhere due to the impacts of pollution and climate change. Cyanobacteria produce a number of cyanotoxins, the most common of which are microcystin-LR and related microcystins, which are potent hepatotoxins. Microcystins are known to bioaccumulate in aquatic organisms, some of which are part of the human food chain. While commercial ELISA kits are available to screen water samples for microcystins and Liquid Chromatography/Mass Spectrometry (LC/MS) methods have been developed to quantitate microcystin in water, there has been limited research on detecting or quantitating microcystins and their metabolites in biological matrices. Some studies have attempted to use commercial ELISA kits developed to screen water samples to screen samples of fish meat, but these methods have not been thoroughly validated. We will develop and validate a robust quantitative LC/MS method to quantitate microcystin in fish, other animal tissues, to be used for veterinary diagnostic investigation and food safety assurance. This method will be used to validate the rapid ELISA-based screening method that has been developed at the University of Wisconsin, Milwaukee.