By adding a small amount of the amino acid cysteine to acidic solution in contact with pyrite, the activity of Thiobacillus ferrooxidans is largely enhanced and the leaching rate of this sulphide is increased three times compared to the normal process without this biochemical additive. In the presence of cysteine, pyrite can also be oxidised in the absence of bacteria with a leaching rate comparable with that attained by bacteria under normal leaching conditions. II appears that the sulphydril group of cysteine participates in a binding process with pyrite. Free-SH groups from the pyrite surface would be the counterpart for the formation of the corresponding disulphide. This thiol-disulphide reaction means that cysteine is consumed by the pyrite surface with the subsequent release of iron-sulfur species. Bacteria would take advantage of this biochemical corrosion process by uptake and oxidation of the released species, which are continuously supplied to the thiobacilli-biotope. The acidic pH value of the leaching solution (below 2.0) favours the binding of reduced cysteine to newly exposed pyritic sulphydryl groups which in turn maintains the ongoing dissolution of pyrite. (C) 2000 Elsevier Science Ltd. All rights reserved.