Tunicamycin inhibited the synthesis of glycosylated mu-chains and kappa-chains in the human lymphoblastoid cell line, Daudi. Nonglycosylated IgM could not be detected on the surface of tunicamycin-treated cells by cell surface iodination techniques even under conditions where membrane IgM was re-expressed in control cultures following the enzymatic stripping of the existing membrane IgM. Biosynthetic labeling and subsequent immunochemical analysis indicated that the nonglycosylated mu and kappa-chains failed to efficiently assemble into monomeric IgM units. In a previous study (Dulis et al., J. biol. Chem., 1982), we have shown that the nonglycosylated mu- and kappa-chains are rapidly catabolized. The lack of expression of nonglycosylated IgM could be due to the rapid catabolism of the nonglycosylated polypeptide chains and/or to the inability to form functional monomeric IgM molecules. Thus glycosylation may be required to protect the newly synthesized polypeptide chains from intracellular catabolic events and to maintain proper conformational foldings of the polypeptide chains to allow for the assembly of subunits into functional units and their ultimate expression.