Technical Abstract: Cercosporin is a perylenequinone pigment produced by fungi in the genus Cercospora which under light generates reactive oxygen species causing membrane damage and mortality of living cells. Our objectives were to evaluate the effects of laccase, a lignolitic copper-containing enzyme on the temporal evolution of the UV-visible spectrum of cercosporin and to demonstrate that laccase can reduce cercosporin toxicity toward living cells. Cercosporin from Cercopsora beticola and Cercospora hayi was treated with laccase from basidiomycete fungi (Pleurotus ostreatus and Trametes versicolor) in the dark and under constant light. Under these conditions the absorbance of the cercosporin decreased at 220, 279 and 295-500 nm within 10 min of reaction with laccase from either P. ostreatus or T. versicolor, indicating that basidiomycete laccase can induce changes in UV-visible spectra of cercosporin. The LIVE/DEAD® Bac LightTM VIABILITY kit and fluorescent microscopy showed more viable E. coli cells after incubation under light with cercosporin and laccase than with cercosporin alone. Lesions were apparent on sugar beet leaves exposed to cercosporin under light after 48 h, but leaves exposed to cercosporin and laccase showed visibly less damage. Furthermore, air-borne microorganisms can grow on cercosporin-producing C. beticola cultures with laccase added under light, but cercosporin-producing cultures without laccase appeared to inhibit their growth. These data suggests that laccase from basidiomycete fungi can decrease the toxic effect of cercosporin toward microorganisms and plant tissue.