Metal effect on the enzyme secretion in fungi is usually related to total concentrations but not to bioavailable metal species. In this work, we aimed at enhancing the secretion of lignin-modifying oxidoreductases in Trametes versicolor by favouring the bioavailability of essential metals. For this purpose, the fungus was exposed to Cu or Zn in liquid culture media exhibiting different complexation levels. Metal speciation was determined experimentally or theoretically to quantify free metal species, supposed to be the most bioavailable, and species complexed to ligands. Although Zn(2+) contents were high in media, Zn had no effect on the oxidoreductase production. Conversely, Cu highly induced the manganese peroxidase and laccase productions until 40 and 310 times when compared to unexposed controls. This inductive potential was highly correlated to Cu(2+) contents in media. Furthermore, in poorly complexing media, the response threshold of oxidoreductases to Cu greatly decreased and an unexpected production of lignin peroxidase occurred. (C) 2010 Elsevier Ltd. All rights reserved.