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The possibility of laccase production by Phanerochaete chrysosporium was studied.A relatively high initial Mn(II) concentration (1–4 mM) in the growth medium leads to the development of reddish-brown coloration and intensive oxidation of 2.2′-azino-bis(3-etilbenz-tiazolin-6-sulfonate) (ABTS). The peak of ABTS oxidation was obtained approximately 1 day after the peak of MnP activity.ABTS oxidation was not caused by manganese peroxidase (MnP) nor by laccase but was the consequence of the action of Mn(III) which was stabilised in the growth medium. Decomposition of the complex took place after the biomass was removed from the growth medium and especially after the aeration of the culture was interrupted.Mn(III) seems to be the cause of false positive laccase reactions. More reliable data on MnP activity can be obtained if the complex is decomposed by the fungus before MnP activity is measured in the medium.