The gel filtration behavior, in the presence of detergents, of membrane-bound IgM from normal mouse spleen B lymphocytes was compared to that of secretory IgM from mouse plasma cells. The proteins were labeled either by surface radioiodination or biosynthetically with radioactive amino acids. Cell lysates were fractionated on calibrated Sepharose 6B columns in the presence of the detergents Nonidet P40 or deoxycholate. Eluted fractions were immunoprecipitated and the reduced or unreduced precipitates were analyzed by sodium dodecyl sulfate gel electrophoresis followed by radioautography. Surface 125I-labeled 8S IgM exhibited a gel filtration pattern in Nonidet P40 corresponding to much higher apparent molecular weight than that of secretory 8S IgM, a difference that almost disappeared when gel filtration was performed in the presence of deoxycholate, which forms much smaller micelles than does Nonidet P40. Biosynthetically labeled lymphocytes contain two types of IgM molecules differing in their gel filtration behavior and fate: one identical to secretory 8S IgM of plasma cells and secreted in the medium during chase periods, and the other identical to surface 25I-labeled IgM and remaining cell-associated. Because the surface-bound 8S IFM was not found to be associated with other labeled molecuJes, it is likely that the detergent-binding behavior of surface IgM is due to a hydrophobic segment carried by these Ig molecules. That lymphocytes synthesize two types of ju chains was also shown by the use of tunicamycin, an inhibitor of glycosylation. In its presence, two unglycosylated y chains were observed: one identical in size to that made by tunicamycin-treated plasma cells, and the second slightly larger. Gel filtration in Nonidet P40 of the cell lysates of tunicamycin-treated lymphocytes showed that the nonsecretory 8S IgM contains this second type of u chains, whereas the IgM mo les of the secretory type contain plasma cell-like ,g chains. It is suggested that membrane IgM us chains contain a hydrophobic segment which is responsible for its association to the membrane.