A hydrogen sulfide:ferric ion oxidoreductase (SFORase), which is crucial in sulfur oxidation of T. ferrooxidans AP19-3, oxidizes hydrogen sulfide with Fe3+ as an electron acceptor to give sulfite ion and Fe2+. When washed intact cells of T. ferrooxidans AP19-3 were treated with hydrogen sulfide solution (100 mM) at pH 6.5 for 1 hr, a heat stable reduced sulfur containing protein, which serves as a reduced sulfur compound for purified SFORase to give sulfite, was formed in the plasma membrane of this strain. The protein, which can bind hydrogen sulfide reversibly, was solubilized from plasma membrane with 1% SDS and purified to an electrophoretically homogeneous state. We tentatively named the protein hydrogen sulfide-binding protein (SBP). SBP had an apparent molecular weight of 16,000 in the presence of 1% SDS. One molecule of SBP had 4.5 molecules of iron and could bind 2.3 molecules of hydrogen sulfide. The hydrogen sulfide bound to SBP was oxidized by a purified SFORase to give sulfite ion, suggesting that SBP is involved in sulfur oxidation of this bacterium.