The white-rot fungus Pycnoporus cinnabarinus, grown on a low nutrient newspaper medium, secreted low amounts of extracellular laccase. The laccase activity was increased when 3% methanol was added to the medium and further increased when 3% dimethylsulfoxide was added. Dimethylsulfoxide increased the enzyme activity over controls when the fungus was grown on a malt extract broth, or media containing either recycled Kraft paper or cellobiose. Treatment with the ionophore A23187 suggested that dimethylsulfoxide acted as a paramorphogen to increase the branch points of the mycelial mat. Hence, the number of ion channels for secretion of the enzyme was increased. When the fungus was grown in a 2 dm 3 bioreactor, sparged with air, the measured extracellular protein concentration rose and remained relatively steady for 15 days. The enzyme activity and speci®c activity reached a maximum between days 6 and 9 and then fell from day 10 of fermentation. The presence of dimethylsulfoxide in the bioreactor caused a four-fold increase in enzyme activity and prolonged its secretion in the bioreactor for at least 25 days. The protein concentration and speci®c activity also remained at high levels over this period.