Cultures of Phanerochaete chrysosporium, operating with an inert carrier (nylon sponge) and a non-inert carrier (barley straw), were employed in order to study laccase production during semi-solid state conditions. Manganese (IV) oxide, added to the cultures increased laccase activity 16-fold especially in barley straw cultures, in which a maximum laccase activity of 360 U/l (one unit is defined as 1 mu mol of 2,2’-azino-di-[3-ethyl-benzothiazoline-(6)-sulphonic acid] oxidized per minute) was achieved.