The white rot fungus Bjerkandera sp. strain BOS55 was shown in previous studies to cause high levels of kraft pulp bleaching and delignification under culture conditions in which manganese peroxidase (MnP) occurs as the dominant oxidative enzyme. In this study, the MnP of Bjerkandera was isolated and tested in vitro with eucalyptus oxygen-delignified kraft pulp (ODKP) based on measuring the reduction in kappa number as an indicator of lignin oxidation. The MnP preparation applied at 60 U/g pulp for 6 h caused a significant decrease of 11-13% in the kappa number in the ODKP under optimal conditions compared to parallel-incubated controls lacking enzyme. The effects of MnP dosage, Mn2+ concentration, organic acid buffer selection, pH and H2O2 addition were evaluated. The optimal Mn2+ concentration range for lignin oxidation in ODKP was 100-500 muM. In the presence of low oxalate concentrations (0.3-2 mM), the Bjerkandera, a MnP also significantly reduced the kappa number of ODKP by 6% without any Mn. This observation is in agreement with the fact that purified Bjerkandera MnP has Mn-independent activities. Under incubation conditions with added Mn2+, buffers composed of metal-complexing organic acids provided two-fold better kappa number reductions compared to the inert acetic acid. The optimal H2O2 dosage was found to be 0.017 mu mol/min ml when added as semicontinuous pulses (every 30 min) or 0.2 mu mol/min ml when generated continuously by glucose oxidase. Excess H2O2 caused severe inactivation of MnP during the incubations. Factors that improved the turnover of the enzyme, such as Mn2+ and metal-chelating acids, stabilized MnP against rapid inactivation. (C) 2001 Elsevier Science Ltd. All rights reserved.