BAP31 is an evolutionarily conserved polytopic integral protein of the endoplasmic reticulum (ER) membrane implicated in regulating the export of selected membrane proteins from the ER to downstream compartments of the secretory pathway. BAP31 interacts with mIgD, cellubrevin, major histocompatibility complex class I, and BCL-2/BCL-X(L) and plays an important role in regulating the egress of these proteins and in apoptosis. Although BAP31 RNA is ubiquitous, the protein's anatomic localization in rat tissues has not been determined. This is partially because production of high affinity antibodies, especially monoclonal antibodies (MAbs) suitable for immunohistochemical staining, has lagged. To gain further insight into its possible functions, we generated a novel MAb specific for rat BAP31 in immunocytochemistry and immunohistochemistry and localized BAP31 in some rat tissues. Immunoreactivity of BAP31 was prominent in fundic glands, colon, pancreatic acinuses, and liver but not in skeleton muscle and lung. Thus, successful production of rat BAP31 monoclonal antibodies provides a new powerful tool for investigation of BAP31 function in the rat model.