The thiol-disulfide oxidoreductase ERp57 is a soluble protein of the endoplasmic reticulum and the closest known homologue of protein disulfide isomerase. The protein interacts with the two lectin chaperones calnexin and calreticulin and thereby promotes the oxidative folding of newly synthesized glycoproteins. Here we have characterized several fundamental structural and functional properties of ERp57 in vitro, such as the domain organization, shape, redox potential, and the ability to catalyze different thiol-disulfide exchange reactions. Like protein disulfide isomerase, we find ERp57 to be comprised of four structural domains. The protein has an elongated shape of 3.4 +/- 0.1 nm in diameter and 16.8 +/- 0.5 nm in length. The two redox-active a and a' domains were determined to have redox potentials of -0.167 and -0.156 V, respectively. Furthermore, ERp57 was shown to efficiently catalyze disulfide reduction, disulfide isomerization, and dithiol oxidation in substrate proteins. The implications of these findings for the function of the protein in vivo are discussed.