A growing list of freshwater cyanobacteria are known to produce toxic agents, a fact which makes these organisms of concern to water authorities. A cultured strain of Limnothrix (AC0243) was recently shown to have toxic effects in invitro bioassays. It did not produce any of the known cyanobacterial toxins. The intrapertoneal toxicity of aqueous extracts of the material was therefore tested in mice to determine whether the observed effects might be of public health relevance to drinking water supplies. The results indicate that Limnothrix AC0243 is acutely toxic to mice, causing widespread cellular necrosis in the liver, kidneys and gastrointestinal tract within 24h of exposure. Sub-lethal effects lasted at least 7d. These results suggest that Limnothrix AC0243 produces a novel toxin ("Limnothrixin") and that further work is therefore urgently required to quantify the potential public health implications.