A novel and promising method of microcystin-LR (mcyst-LR) degradation is reported. The decomposition of this cyanobacterial toxin using Fenton reagent has been observed with very low initial concentrations of H 2 O 2 and Fe 2 + (Fe 3 + ) in the reaction mixture. Mcyst-LR was isolated from a laboratory culture of Microcystis aeruginosa PCC 7813. The initial concentration of the toxin used exceeded by several orders of magnitude those occurring naturally in lakes and drinking water. Even so, the decomposition of the toxin was complete after 30min.