Glucose regulated protein 78 (GRP78) is a member of the heat shock protein (hsp) 70 family. It is an endoplasmic reticulum (ER) chaperone, whose function is generally thought to be limited to the structural maturation of nascent glycoproteins. However, recent observations have shown that ER chaperones, such as GRP78, display peptide-binding activity. These peptide-binding activities along with the observation that heat shock proteins associated with peptides can elicit antigen-specific CTL responses suggest additional roles for these proteins. In this study we provide evidence that GRP78 is not only resident in the ER, but also exists on the cell surface. Furthermore, using biochemical and imaging studies we have found that GRP78 associates with MHC class I on the cell surface. Its presence on the cell surface is not dependent on MHC class I expression. In the absence of MHC class I its cell surface expression is upregulated.