More than ten years ago a heavy chain binding protein (BiP) was described which is associated with immunoglobulin heavy chains (HC) within the endoplasmic reticulum (ER) [which is the site of Immunoglobulin (Ig) assembly]. Recently, Linda Hendershot and her collegues suggested that BiP might combine with nascent HC as they enter the ER and hold them there until assembly with light chain (LC) occurs. In the absence of LC synthesis or assembly, the HC would remain associated with BiP and would eventually be degraded internally. They now propose a means for BiP to block the transport of unassembled Ig molecules. Transport of protein from ER to the Golgi apparatus seems to be mediated by transport signals inherent to the protein molecule itself. Ig transport signals have been thought to be on the LC because LC can be secreted alone while HC cannot under normal circumstances. When BiP is displaced by LC, completed Ig molecules are transported. They use this model to explain regulated transport of Ig molecules during B-cell development, and suggest that BiP may be post-translationally associated with the nascent chains of other membrane and secretory proteins before folding or subunit assembly. © 1987.