We have investigated the importance of glycosylation in determining the function of membrane-bound and secreted immunoglobulin M (IgM). Hickman and Kornfeld (1978) previously observed that glycosylation is required for IgM to be secreted by 104E, a mouse plasma cell tumor. In order to determine whether this requirement is a general one for all forms of IgM, we have used WEHI 279.1, a mouse B lymphoma that synthesizes both the membrane and secreted forms of IgM. In the presence of 5 microgram/ml tunicamycin (Tm), glycosylation of both membrane and secreted IgM is at least 90% inhibited, but total protein synthesis is equivalent in control and Tm-treated cells. Despite the absence of carbohydrate, IgM molecules are properly assembled into monomers for membrane localization. Cells whose surfaces have been stripped of membrane IgM by treatment with anti-mu antibody resynthesize the IgM equally well in the presence or absence of Tm. It is more surprising that the assembly of IgM into pentamers and the secretion of these pentamers into the medium are accomplished at the same rate and to about the same levels in control and Tm-treated WEHI 279.1 cells. This is in sharp contrast to the profound inhibition of IgM secretion observed when the plasmacytoma cell 104E is treated with the same concentration of Tm (5 microgram/ml). Although both WEHI 279.1 and 104E cells secrete IgM, the 2 are models for cells at very different points along the B cell differentiation pathway. The difference in the effect of Tm-treatment on IgM secretion may reflect a cellular change that occurs during this differentiation. The unglycosylated IgM is very sensitive to trypsin digestion, whereas the native forms are not. This suggests that the function of glycosylation may be to stabilize the IgM against proteolysis.