The production of MnP by Pleurotus ostreatus in different liquid cultures was investigated. The highest level of activity was observed after 8 days of culture in peptone-glucose-yeast extract medium (PGY), whereas maximal activity was achieved after 30 days in glucose-yeast extract medium (GY). MnP was purified to homogeneity from PGY (designated MnP-PGY) and GY (MnP-GY). The isoelectric points of MnP-PGY and MnP-GY were 3.77 and 4.06, respectively. The molecular mass of both enzymes was 42 kDa. Analysis of the N-terminal amino acid sequence of purified MnPs and nucleotide sequence of cloned mnp indicated that MnP-GY has VTCATGQTTANE at the N-terminus, whereas MnP-PGY has ATCADGRTTANA. A putative exposed tryptophan residue (W170) was found in MnP-GY. Both isozymes oxidized veratryl alcohol, although the K(m) of MnP-GY was lower than that of MnP-PGY. Thus, the presence of peptone in the medium affected the production of MnP isozymes. Reverse transcription-polymerase chain reaction (RT-PCR) analysis indicated that the synthesis of MnP isozymes is controlled by culture conditions at the transcriptional level.