This study provides a first attempt from a toxicological perspective to put forward, in general terms and explanations, the toxicity series of Cd(II), Cu(II) and Zn(II) to Thiobacillus thiooxidans BC1. Sulphur oxidation and sulphuric acid production are strongly related to microbial growth at pH less than 4. Dose-response analysis on chronic and acute toxicity (e.g. EC20, median effective dose EC50 and slope factor B) of divalent cadmium, copper and zinc cations suggests a toxicity series of Cu > Cd >> Zn to T thiooxidans BCl. Zn(II) is termed non-toxic and the maximum treatment concentrations of Cd(II) and Cu(II) are approximately 300, 400 mg/l, respectively. This assessment clearly indicates viable operation ranges of metal bioleaching for mine wastewater treatment, suggesting a technological feasibility of biotreatment using acidophilic thiobacilli T thiooxidans BC1. (C) 2003 Elsevier Ltd. All rights reserved.