The potential for the oxidant ozone used in water treatment, to destroy the various classes of toxins produced by cyanobacteria (blue-green algae) was investigated. Toxin destruction was determined by chemical analyses or mouse bioassay. Ozone was more effective than chlorine, hydrogen peroxide and potassium permanganate in destroying the peptide hepatotoxin microcystin-LR. Ozone in combination with hydrogen peroxide was more effective than ozone alone. Ozone was capable of destroying the alkaloid neurotoxins anatoxin-a and paralytic shellfish poisons (PSPs), whereas chlorine has been previously reported to be ineffective (Rositano and Nicholson, 1994). Destruction of peptide hepatotoxins by ozone was pH-dependent. Ozone was less effective under alkaline conditions, consistent with its lower oxidation potential under these conditions (1.24V) compared with acidic conditions (2.07V).