The toxicity of Co(II), Mn(II), Cd(II), and Zn(II) for Pseudomonas aeruginosa PU21, a Hg(II)-hyperresistant strain containing the mercury resistance mer operon, was determined. The metal tolerance of PU21 was strongly influenced by environmental conditions (e.g., existing metal, medium composition). Dose-response analysis on chronic and acute toxicity (e.g., EC20, median effective dose EC50, and slope factor B) of divalent cobalt, manganese, cadmium, and zinc cations in LB medium amended with citric acid phosphate buffered saline (CAPBS) suggested a toxicity series of Co > Mn approximate to Zn > Cd for EC50. In contrast, excluding the likely precipitate of Zn(II), the toxicity ranking in phosphate-buffered saline (PBS)-amended LB medium was Co > Cd > Mn. The metal toxicity in PBS, irrespective of metals, was greater than that in CAPBS. This might be attributed to the presence of citric acid in CAPBS as a chelating ligand donating electrons to hold free metals (e.g., Cd2+, Zn2+ tetrahedral ML4 complex). The toxicity assessment established viable operation ranges (ca. < EC50-EC70) to prevent operation failures for metal detoxification and biosorption of mine wastewater treatment by P. aeruginosa PU21. (c) 2005 Elsevier Ltd. All rights reserved.