Solubilization of Cu2+ from copper concentrate by 67 strains of iron-oxidizing bacteria isolated from the natural environment was studied. In the case of static growth on copper concentrate (5%)-salt medium (pH 2.5) at 30 degrees C, Cu2+ solubilization activities of 65 strains including Thiobacillus ferrooxidans strains AP19-3, OK-2, OK-3, OK1-50, Funis and NASF-1 ranged from 2.5-3.5 mg Cu2+/ml/33 d. in contrast, the activities of T. ferrooxidans strains KO-1 and NA-1 were quite low at 0.9 mg Cu2+/ml/33 d. Nonbiological solubilization activity of Cu2+ was 0.4 mg Cu2+/ml/33 d. For identification of the enzymes that play a crucial role in biological Cu2+ solubilization, the activities of the enzymes involved in iron and sulfur oxidations were compared among the strains described above. No significant differences in the growth on iron-salt medium and the levels of cellular iron oxidizing activity were observed, In contrast, a marked difference was observed in the cellular activity of an enzyme that was involved in the first step of sulfur oxidation in T. ferrooxidans, i.e., hydrogen sulfide : ferric ion oxidoreductase (SFORase). Strains Funis, OK1-50, NASF-1, OK-3, AP19-3, OK-2 which showed high Cu2+ solubilization activity showed high SFORase activity (1.3-4.0 mu mol Fe2+/ml/4 h). In contrast, strains NA-1 and KO-1 which showed low Cu2+ solubilization activity showed low SFORase activity (0.3-0.4 mu mol Fe2+/ml/4 h), suggesting that SFORase, but not iron oxidase, is the enzyme that determines the ability of T. ferrooxidans to solubilize Cu2+ from copper concentrate.