The murine B cell line cloned from a single cell, 38C‐13, synthesizes three species of μ chains, that of cell surface membrane IgM (m‐μ), that of secreted IgM (s‐μ) and that of intracellular IgM (i‐μ). They differ in their mobility on sodium dodecyl sulfate polyacrylamide gel electrophoresis. Sequence analysis of the different μ chains suggests that they are identical in the N‐terminal as well as in their C‐terminal positions. The ratio between incorporated radioactive monosaccharides to radioactive amino acids into the three different μ chains was higher in s‐μ than in m‐μ, but nevertheless m‐μ migrated more slowly than s‐μ on sodium dodecyl sulfate polyacrylamide gel electrophoresis. However, since this ratio may also be influenced by the rate of synthesis, it may not represent a real molar ratio of carbohydrate to protein. Studies with normal spleen cells clearly indicated the presence of the same three types of μ chains.