A mixed culture of Thiobacillus ferrooxidans, T. thiooxidans, and Leptospirillum ferrooxidans was used for inoculation of a sulfidic ore body for a bacterial in-situ stope-leaching expt. in the Ilba mine in Romania. The ore body was inoculated with 107 cells/g ore. Measurements at six main sites of the ore body indicated that microbial leaching was started by the inoculation. After about 8 wk sufficient microbial activity was measurable only in the upper third of the ore body. Due to the angle of incidence of the ore (75 Deg), the leach liquor percolated only through the upper part leaving two-thirds humidified unsatisfactorily. The leach results, metal mobilization, indicated that by inoculation with the indigenous microorganisms efficient leaching was achieved. Metal output after 18 mo of operation amounted to 10% of Cu and 78% of Zn. In the winter months energy for aeration and circulation was not available and this was reflected by reduced values for microbial activity, temp., and daily metal output. The biol. metal mobilization after 18 mo of operation was as active as at the beginning. Cu was mobilized predominantly by microbial leaching whereas Zn was leached mainly by chem. reactions. Both mechanisms contributed equally to iron output.