A study was conducted on the ability of laccase from Trametes versicolor to catalyse the oxidation of triclosan, an antibacterial agent of significant commercial importance and environmental concern. Laccase was able to catalyse the transformation of triclosan under a variety of conditions and achieve a substantial decrease in toxicity of the reaction mixture. The optimal pH for triclosan transformation was approximately S, with a broad optimum in the range 4-6. Treatment could be achieved at elevated temperatures, but at the expense of higher rates of inactivation. Treatment efficiency and reaction rates were substantially improved through the use of a protective additive, poly(ethylene glycol) (PEG-35000), and a mediator, 2,2'-azino-bis(3-ethylbenzthiazoline)-6-sulfonic acid (ABTS). However, both compounds negatively affected the toxicities of treated solutions. The presence of ions including sulfite, sulfide, cyanide, chloride, iron(III) and copper(II) resulted in reduced treatment efficiency. (c) 2006 Society of Chemical Industry.