The immunoglobulin (Ig) molecule is composed of two identical heavy chains and two identical light chains (H₂L₂). Transport of this heteromeric complex is dependent on the correct assembly of the component parts, which is controlled, in part, by the association of incompletely assembled Ig heavy chains with the endoplasmic reticulum (ER) chaperone, BiP. Although other heavy chain-constant domains interact transiently with BiP, in the absence of light chain synthesis, BiP binds stably to the first constant domain (C_H1) of the heavy chain, causing it to be retained in the ER. Using a simplified two-domain Ig heavy chain (V_H-C_H1), we have determined why BiP remains bound to free heavy chains and how light chains facilitate their transport. We found that in the absence of light chain expression, the C_H1 domain neither folds nor forms its intradomain disulfide bond and therefore remains a substrate for BiP. In vivo, light chains are required to facilitate both the folding of the C_H1 domain and the release of BiP. In contrast, the addition of ATP to isolated BiP–heavy chain complexes in vitro causes the release of BiP and allows the C_H1 domain to fold in the absence of light chains. Therefore, light chains are not intrinsically essential for C_H1 domain folding, but play a critical role in removing BiP from the C_H1 domain, thereby allowing it to fold and Ig assembly to proceed. These data suggest that the assembly of multimeric protein complexes in the ER is not strictly dependent on the proper folding of individual subunits; rather, assembly can drive the complete folding of protein subunits.