A class of toxins produced by cyanobacteria (blue-green algae), known as microcystins, are cyclical heptapeptides that cause lethal hepatic failure in domestic and wild animals that ingest contaminated stagnant water throughout the world. Microcystins are specifically transported into hepatocytes where they irreversibly bind to and inhibit protein phosphatases. This is the first report of toxicity in humans and was caused by contamination of the water supply of a dialysis unit. 101 of 124 patients at risk developed liver abnormalities and 50 died of acute liver failure. Microcystins were found in the municipal water supply and the water used for dialysis. Another dialysis unit received water from the same source, but it had been properly treated; no microcystins were found and the unit's patients were free of liver failure. This report of human parenteral exposure to microcystins underscores the importance of proper water treatment and the potential danger to humans exposed to natural environmental hepatotoxins.