Ophiostoma ulmi and O. novo-ulmi, both causative agents of Dutch elm disease, can be differentiated by their potential to produce constitutive extracellular laccase. The enzyme has been purified from the culture filtrate to apparent electrophoretic homogeneity and has been partially characterized. The laccase was glycosylated and found to have a molecular mass of 79 kDa or 70 kDa by SDS-PAGE and gel filtration, respectively. The pI, determined by chromatofocusing, was 5.1. Syringaldazine, guaiacol, and other typical laccase substrates were oxidized. No oxidation of tyrosine was detected. NaN3 (0.01%) completely abolished the activity towards 2,6-dimethoxyphenol.