Oligomeric antigen receptors must fold and assemble in the endoplasmic reticulum (ER) before they can be expressed on the surface of lymphocytes. It is increasingly evident that these processes are facilitated by molecular chaperones. Here, Jeffrey Melnick and Yair Argon review the known ER chaperones, summarize their roles in the maturation of antigen receptors, and discuss how they may affect lymphocyte differentiation and function.