Glyceryl trinitrate (GTN) 2 mM was quantitatively converted into its 1 and 2 mononitrate derivatives by Geotrichum candidum, with consumption of the nitrate ions produced. The conversion proceeded at a rate independent of the addition of either organic carbon or organic nitrogen sources. Eight batches of nitrate ester, which were added every 24 hours, were successfully converted as far as during the bioconversion process GTN concentration did not exceed 2 mM. When those limiting conditions were not observed, dramatic toxivity of GTN was noticed.