The white-rot fungus Pleurotus ostreatus was used by a company for the bioremediation of PAH-contaminated site near Hamburg (FRG). The soil material was supplemented with a white-rot fungus/straw-material. The efficiency of this practical bioremediation was monitored in a scientific research program (WUP Veringstrasse) for > 2 yr. In this program, it was our part to investigate the mass balance of the PAH degradation in more detail in laboratory experiments Mass-flow experiments were carried out with 14C-labeled PAH. We were able to measure complete mass balances with a recovery rate of 100±7%. It was shown by comparison with sterilized soil samples that the observed 14C-PAH degradation in soil could clearly be correlated with biol. activities. The mineralization rate (given as %14C-CO2 of the original radioactivity) ranged between < 5% for benzo(a)pyrene and up to 70% for 14C-naphthalene in the biol. active soil material, but was < 1% 14C-CO2 for all 14C-PAH under sterile test conditions. Control experiments without supplementation of the white-rot fungus/straw-material showed that the 14C-PAH degradation rate could not significantly be raised by the addition of Pleurotus ostreatus. It was found that in biol. active soil material, a relevant part of the residual activity in soil was not extractable after incubation. The extent of the nonextractable residues ranged between 45% of the original radioactivity in experiments with 14C-anthracene and 25% in experiments with 14C-naphthalene. The formation of nonextractable residues was generally higher in biol. active soil than in sterile soil. The formation of nonextractable residues seems therefore to be at least partly due to a biol. process. Experiments on the remobilization of these nonextractable residues showed that after renewed supplementation with fungus/straw-material neither a significantly higher mineralization nor a remobilization of nonextractable residues could be measured.