In the presence of phosphate ions, molybdic ions (Mo6+) were reduced enzymatically with elemental sulfur by washed intact cells of Thiobacillus ferrooxidans to give molybdenum blue. The whole-cell activity that reduced Mo6+ was totally due to cellular sulfur:ferric ion oxidoreductase (SFORase) (T. Sugio, W. Mizunashi, K. Inagaki, and T. Tano, J. Bacteriol. 169:4916-4922, 1987). The activity of M06+ reduction with elemental sulfur was competitively inhibited by Fe3+, Cu2+, and Co2+. The Michaelis constant of SFORase for Mo6+ was 7.6 mM, and the inhibition constants for Fe3+, Cu2+, and Co2+ were 0.084, 0.015, and 0.17 mM, respectively, suggesting that SFORase can reduce not only Fe3+ and Mo6+ but also Cu2+ and Co2+ with elemental sulfur.