Microcystis aeruginosa produces toxic cyclic peptides called microcystins, potent hepatotoxins that have been implicated in tumor promotion in skin and liver. The model used in this investigation was the azoxymethane (AOM)-induced aberrant crypt focus (ACF) in the male C57Bl/6J mouse colon. Three intraperitoneal (ip) injections of 5 mg/ kg AOM were administered at 7-d intervals to mice; 19 d after the last AOM injection, drinking water containing Microcystis extract was commenced and continued for a further 212 d. The content of microcystins in the drinking water was determined by mouse bioassay, high-performance liquid chromatography (HPLC), capillary eletrophoresis, and protein phosphatase inhibition. The doses employed were 0, 382, and 693 µg/kg bodyweight/d at the midpoint of the trial. Following postmortem examination blood cells, serum enzymes and organ pathology were investigated. A significant microcystin dose-dependent increase in the area of aberrant crypt foci was observed. There was no marked increase in the number of crypts/colon. Two overt colonic tumors (~30 mm3) were seen in microcystin-treated mice, and one microscopic colonic tumor in an AOMalone-treated mouse. This investigation provides the first evidence for the stimulation of preneoplastic colon tumor growth by microcystin.