The human lymphoma cell line Daudi has the phenotype of a nonsecreting B cell. This cell line synthesizes both the membrane and secreted forms of the IgM heavy chain, but only expresses functional membrane IgM. We have found that secreted type heavy chains (mus) are rapidly degraded in these cells, with a half-life of 1.3 h. Some of the membrane type heavy chains (mum) are also rapidly catabolized but some are expressed in a stable form with a half-life of 13 h. Inhibiting the initial glycosylation of heavy chains with tunicamycin has differential effects on the catabolic rates of mus and mum chains. The turnover of mus chains is not affected by this inhibitor, but the degradation of mum chains is much more rapid after tunicamycin treatment. In comparison with their glycosylated counterparts, nonglycosylated mu chains do not covalently assemble to a significant degree with light chains. Tunicamycin treatment of Daudi cells thus seems to inhibit formation of stable mum protein, possibly by altering mu chain conformation and inhibiting its interaction with light chains. We conclude from these results that some mum chains are specifically protected from proteolysis by post-translational events. These processing events include covalent assembly with light chains, terminal glycosylation, and insertion into the plasma membrane.