Bio-oxidation of refractory gold concentrates is currently the alternative of choice as a pretreatment stage prior to cyanidation in gold mining, Bacterial attack removes part or the entire sulphide layer that covers the gold microparticles, facilitating the gold extraction. Common sulphides found in these minerals are pyrite and arsenopyrite, while enargite is rarely found. The objective of this work was to set up a continuous bio-oxidation system for a concentrate of high enargite and pyrite content and to evaluate the effect of the hydraulic residence time (3-8 days) and the pulp density (6-24%) on the process. Results show that the bacterial attack was directed mainly to pyrite as suggested by extractions of 32% for iron (pyrite) and only 2.4% for arsenic (enargite). Increasing residence times had a positive effect on bio-oxidation, while high pulp densities had an adverse effect. (C) 2002 Elsevier Science Ltd. All rights reserved.