Endoplasmic reticulum oxidoreductases (Eros) are essential for the formation of disulfide bonds. Understanding disulfide bond catalysis in mammals is important because of the involvement of protein misfolding in conditions such as diabetes, arthritis, cancer, and aging. Mammals express two related Ero proteins, Ero1α and Ero1β. Ero1β is incompletely characterized but is of physiological interest because it is induced by the unfolded protein response. Here, we show that Ero1β can form homodimers and mixed heterodimers with Ero1α, in addition to Ero-PDI dimers. Ero-Ero dimers require the Ero active site, occur in vivo, and can be modeled onto the Ero1p crystal structure. Our data indicate that the Ero1β protein is constitutively strongly expressed in the stomach and the pancreas, but in a cell-specific fashion. In the stomach, selective expression of Ero1β occurs in the enzyme-producing chief cells. In pancreatic islets, Ero1β expression is high, but is inversely correlated with PDI and PDIp levels, demonstrating that cell-specific differences exist in the regulation of oxidative protein folding in vivo. © 2005 by The American Society for Biochemistry and Molecular Biology, Inc.