The growth of thiobacilli on sterilized sulfide ore (Rammelsberg, FRG) in glass percolators was investigated under various conditions. The percolators were inoculated with 108 cells/g ore of Thiobacillus ferrooxidans, T. intermedius/T. novellus, T. neapolitanus, and T. thiooxidans, resp. One expt. was started at a pH-value of 4.5 without further pH-adjustment. The value increased to pH 6.5 due to the buffering capacity of the ore. After 120 days the cell counts had fallen to <106 cells/g ore. Solubilization of metals was not detectable. The second expt. was run on a pH-value of 2.5 with pH-adjustment by chem. acidification. After approx. 100 days about 0.1 mol/kg ore of sulfuric acid (2N) had been added. The cell nos. of the thiobacilli remained at the level of inoculation. Metal solubilization was obsd. The third expt. dealt with pH-adjustment by biol. acidification. It was started at a pH-value of 3.5. Instead of sulfuric acid sterilized sulfur was added. This resulted in a rapid decrease in pH. Within 28 days a pH-value of 2 due to biol. sulfuric acid prodn. was measured. The cell nos. of T. thiooxidans and T. ferrooxidans increased to about 1010 cells/g ore. An increase in metal mobilization was detected. Thus, without sulfuric acid addn. poor leaching of the sulfide ore was obtained, with sulfuric acid appreciable leaching occurred, and with sulfur addn. the best leaching results were achieved.