AbstractUsing cultured mouse myeloma cells, it has been possible to derive cells which are now synthesizing products similar to human heavy chain disease proteins. An initial mutant was isolated which synthesized a heavy chain with an internal deletion and a normal light chain. In a subsequent step, a variant was identified in which the synthesis of the heavy chain with the deletion persisted in the absence of light chain synthesis. These experiments also demonstrate that in the MPC‐11‐derived cultured cell line, the continued synthesis of heavy chain does not require the synthesis of light chain.