AbstractWe have investigated immunoglobulin production by cloned hybrid cells. γ2b heavy chains, but not γ1 or γ2a, exist in two forms, differing in molecular weight, galactose labeling and charge. Hybrid molecules containing γ2b and γ1 heavy chains were secreted by hybridoma cells synthesizing Ig molecules of both subclasses. However, cells synthesizing both IgM and IgG chains did not secrete μ‐γ hybrid molecules. We have surveyed many new hybridomas derived from different mouse strains: 10 % of those secreting IgM produce a μ chain which is about 5000 daltons larger than normal μ. Two IgM‐secreting lines were studied in detail and were found to bear μ but not δ in their membrane. Intra‐ and extracellular μ differ in molecular size; pentameric IgM‐containing molecules of both sizes were observed in the membrane fraction.