KDEL (i.e., Lysine-Aspartate-Glutamate-Leucine) is a tetrapeptide motif that serves as a retention sequence at the carboxy-terminus of proteins which reside within the ER lumen. A specific receptor located in or near the cis-Golgi recognizes proteins bearing carboxyterminal KDEL and mediates their return to the ER. We reasoned that it may be possible to saturate the KDEL receptor with a hapten, thereby competitively inhibiting its function in living cells. We hypothesized that if we blinded the receptor with an excess of hapten then the intermediate compartment should be unable to differentiate between polypeptides containing carboxyterminal KDEL from those that do not. If the receptor could be blinded, then KDEL-polypeptides such as BiP should be secreted.