The proper folding and assembly of major histocompatibility complex (MHC) class I molecules in the endoplasmic reticulum (ER) is an intricate process involving a number of components. Nascent heavy chains of MHC class I molecules, translocated into the ER membrane, are rapidly glycosylated and bind the transmembrane chaperone calnexin. In humans, after dissociation from calnexin, fully oxidized MHC class I heavy chains associate with beta 2-microglobulin (beta 2m) and the soluble chaperone calreticulin. This complex interacts with another transmembrane protein, tapasin, which is believed to assist in MHC class I folding as well as in mediating the interaction between assembling MHC class I molecules and the transporter associated with antigen processing (TAP). The TAP heterodimer (TAP1-TAP2) introduces the final component of the MHC class I molecule by translocating peptides, predominately generated by the proteasome, from the cytosol into the ER where they can bind dimers of beta 2M and the MHC class I heavy chain. Recently, the thiol oxidoreductase ERp57--also known as GRP58, ERp61, ER60, Q2, HIP-70, and CPT and first misidentified as phospholipase C-alpha--has been shown to bind in conjunction with calnexin or calreticulin to a number of newly synthesized ER glycoproteins when their N-linked glycans are trimmed by glucosidases I and II. It was speculated that ERp57 is a generic component of the glycan-dependent ER quality control system. Here, we show that ERp57 is a component of the MHC class I peptide-loading complex. ERp57 might influence the folding of MHC class I molecules at a critical step in peptide loading.