In 2001, a University of California, Davis-University of the Republic, Montevideo, partnership created a Fogarty ITREOH program to exploit the potential of ELISA to provide a low-cost environmental analysis attractive to economically distressed countries of temperate South America. This paper describes the development and validation of an ELISA method for the determination of Cyanobacteria microcystin toxins in algal blooms, which release hepatotoxic metabolites that can reach toxic levels in rivers, lakes, or coastal estuaries used for recreation or water supplies. The assay made possible the first systematic monitoring of water from the Rio de la Plata at Montevideo over two summers. The project has been integrated into a bi-national effort to monitor the Rio de la Plata.