E. acidaminophilum is a strict anaerobic, gram positive, rod-shaped bacterium, which belongs to the cluster XI of clostridia. It ferments amino acids in a stickland type of reaction. Glycine can be used as the only source of carbon and energy. E. acidaminophilum possesses at least eight selenocysteine-containing proteins. One of these, PrxU, is a protein with a molecular mass of 22 kDa and significant homologies to members of the peroxiredoxin family. These kind of proteins belongs to an enzyme system, that catalyzes the reduction of hydrogen peroxide and organic hydroperoxides. Crude extract of E. acidaminophilum exhibits a corresponding enzyme activity by using NADPH or DTT as electron donor. By purification of the DTT-dependent activity, three proteins (47, 25, and 22 kDa) were obtained, which corresponded to GrdB and the processed GrdE forming together protein B of glycine reductase. The purified proteins were biochemically characterized. The selenocysteine as well as at least one of the neighboring cysteines (UxxCxxC) in GrdB are involved in catalysis of peroxide reduction. GrdE seemed to have chaperon-like activity toward GrdB. The NADPH-dependent activity could not be restored by adding thioredoxin, thioredoxin reductase, GrdA and the putative redox-active protein PrpU. The selenoperoxiredoxin PrxU was heterologously synthesized in Escherichia coli and following purified. It exhibited DTT-dependent enzyme activity in the presence of linoleic acid hydroperoxide. The selenocysteine was essential for activity. As native electron donor a unknown bio-thiol was postulated.