The extracellular fluid of ligninolytic cultures of the white-rot wooddestroying fungus,Phanerochaete chrysosporium Burds., contains an enzyme that degrades lignin model compounds as well as lignin itself (1). Like ligninolytic activity, the enzyme appears during idiophasic metabolism, which is triggered by nitrogen starvation. The enzyme has been purified to homogeneity by DEAE-Biogel A chromatography, as assessed by SDS polyacrylamide gel electrophoresis, isoelectric focusing, and gel filtration chromatography. These techniques also revealed a molecular weight of 42,000 daltons, and an isoelectric point of 3.4. The purified enzyme exhibits low substrate specificity. It is an oxygenase, but requires hydrogen peroxide for activity. The activity is optimum at 0.15 mM H2O2; at concentrations above 0.5 mM, H2O2is inhibitory. Model compound studies have shown that the enzyme catalyzes cleavage between Cα and Cs in compounds of the type aryl-CαHOH—CsHR-(R = -aryl or -O-aryl), and in the Cα-hydroxyl-bearing propyl side chains of lignin. This cleavage produces an aromatic aldehyde moiety from the Cα-portion, and a Cs-hydroxylated moiety from the Cα-portion. Cleavage between Cα and Cs in arylglycerol-Β-aryl ether structures leads indirectly to cleavage of the Β-aryl ether linkage, which is the most abundant intermonomer linkage in lignin. The Cs-hydroxyl oxygen comes from molecular oxygen, and not from H2O2, as determined by18O isotope studies. The pH optimum for these reactions is between 2.5 and 3.0; no activity is observed above pH 5. Formation of the expected aldehydes from spruce and birch lignins, and partial depolymerization of the lignins results from the action of the purified enzyme. In addition to Cα—Cs cleavage, the enzyme catalyzes aromatic alcohol oxidation, aryl methylene oxidation, hydroxylation at Cα and Cs in models containing a Cα—Cs double bond, intradiol cleavage in phenylglycol structures, and phenol oxidations.