A cloned cell line derived from mouse bone marrow and transformed by Abelson virus is shown to synthesize two different heavy chains, μ and γ(2B), in vitro. This characteristic is stable because it persists upon subcloning. Although most of the immunoglobulin-synthesizing cells produce either μ or γ(2B) heavy chains, a few cells contain both heavy chains, suggesting immunoglobulin class switching. Karyotypes show a complement of 41 chromosomes. Two copies of chromosome 12, to which immunoglobulin heavy chain structural genes have been assigned, were found. No light chain was found in either the μ- or the γ(2B)-producing cells. However, fusion of the cell line with a myeloma that synthesizes neither heavy nor light chains caused expression of κ light chain in the hybridomas synthesizing μ chain. No light chain could be detected in the hybridomas synthesizing γ(2B) heavy chain.