Abstract: Acid mine drainage (AMD) is one of the most serious environmental problems in the metal-mining industry. AMD is formed by the chemical and bacterial oxidation of sulphide minerals, and it is characterized by low pH values and high sulphate and metals content. The most common method to treat AMD is chemical neutralization. The chemical treatment requires high capital and operating costs and its use is problematic at the closed mines sites. Outokumpu has studied and used sulphate reducing bacteria (SRB) as an alternative method for the treatment of AMD. SRB existing in many natural anaerobic aqueous environments can reduce sulphate to sulphide which precipitates metals as extremely insoluble metal sulphides. Full scale experiments were begun in summer 1995 in the Ruostesuo open pit (depth 46 m) by adding liquid manure as a source of bacteria and press-juice as a growth substrate. The average Zn content of the whole column has decreased from 3,5 mg/l to 0,8 mg/l and below 25 m zinc is 0 mg/l. Similar results have been reached with nickel in the Kotalahti old nickel mine, where bacteria were brought in 1996. We have found that the same bacterial mechanism acts in peat-limestone filters, which Outokumpu has built at several mine sites since 1993.