Here we show that not only transport defective but all immunoglobulin light chains interact with BiP. Association of BiP with its ligand takes place during or shortly after translation of the light chains. The biological half life of the BiP-light chain complex depends on the fate of the light chains. Light chains which are secreted interact with BiP for only a very short time. In contrast, the complex is biologically more stable in cells which do not secrete their L chains. In these cells, dissociation from BiP correlates with the biological half life of the L chains arguing for a degradation pathway in the endoplasmic reticulum. Instead of being degraded in association with its ligand, BiP is released from the complex and binds to newly synthesized polypeptides. These results support the notion that both H and L chains require the chaperoning function of BiP before or during the process of antibody assembly.