The degradation of the postharvest fungicide imazalil by the oxidative enzyme laccase was investigated in the presence of a reaction mediator (a laccase/mediator system). Of various natural mediators tested, imazalil was degraded most effectively in the presence of 4-hydroxybenzoic acid while the absence of a mediator resulted in no degradation. In this system, the mediator compound was oxidized by laccase and then the oxidized mediator oxidized imazalil. The study on the cytotoxicity of the degraded-imazalil solution against mouse fibroblast L929 cells revealed toxicity of 4-hydroxybenzoic acid. The cytotoxic study also suggested that instead of 4-hydroxybenzoic acid, the used of methyl 4-hydroxybenzoate as the reaction mediator reduced the intrinsic cytotoxicity of imazalil. (c) 2006 Elsevier Ltd. All rights reserved.