A mixed culture of Thiobacillus (T.) ferrooxidans, T. thiooxidans, and Leptospirillum (L.) ferrooxidans was used for inoculation of an ore body for a bacterial in-situ stope leaching expt. in the Ilba mine in Romania. The ore body was inoculated four times successively with finally 106 cells/g ore. Cell counts, microbial activity (measured by microcalorimetry), pH and redox values, metal concns., and temp. were measured at 16 sites. Microbial leaching started with the inoculation. Only the upper third of the ore body was leached satisfactorily because of the angle of incidence of the ore (75 Deg) and the leach installations. Metal output after 24 mo amounted to 10% Cu and 78% Zn although during the winter energy for aeration and circulation was not available. Bioleaching contributed most to Cu solubilization and least to Zn output. The microbiol. metal mobilization rates indicate that after 2 yr the microorganisms solubilized metals at nearly the same rate as at the beginning. L. ferrooxidans was as important as T. ferrooxidans. Lab. expts. established the growth conditions required. Most important are temps. above 20 DegC and high humidity. This was corroborated by the results of a field study of dumps near Cartagena, Spain. Further lab. expts. showed that only leaching with L. ferrooxidans is enhanced by the presence of chemoorganotrophic bacteria. This may result from increased exopolymer prodn. furthering attachment and biofilm formation.