Lignin is a complex polymer of phenylpropanoid subunits. It is an essential component of woody tissue, to which it imparts structural rigidity. Lignin is remarkably resistant to degradation by most microbes; nevertheless, a few species of white-rot fungi are able to catalyse its oxidation to CO2. Its biodegradation is of great ecological significance because, next to cellulose, lignin is the most abundant renewable polymer on Earth. The first step in lignin degradation is depolymerization, catalysed by the lignin peroxidase isozymes (ligninases). These isozymes are secreted, along with hydrogen peroxide (H2O2) by the fungus Phanerochaete chrysosporium Burds, under conditions of nutrient (nitrogen) limitation. Ligninases are not only important in lignin biodegradation, but are also potentially valuable in chemical waste disposal because of their ability to degrade environmental pollutants. We have undertaken the cloning of the ligninase genes to understand further their regulation and enzymology. We report here the isolation and characterization of a ligninase complementary DNA clone with a full-length insert. The cDNA sequence shows that the sequence of the mature ligninase is preceded by a 28-residue leader, and the mature protein is predicted to have a relative molecular mass of 37,000 (Mr 37K). Consistent with the classification of ligninase as a peroxidase certain residues thought to be essential for peroxidase activity can be identified and near these residues the ligninase shows homology with other known peroxidases. Our cDNA clone has also allowed us to show that expression of ligninase is regulated at the messenger RNA level.