Bone marrow cells were obtained from a patient with gamma heavy chain disease (HCD) whose serum contained a deleted immunoglobulin heavy chain. Incubation of the marrow cells with radioactive amino acids in short term tissue culture resulted in the synthesis of the labelled HCD protein. A permanent cell line was established from the peripheral blood of the patient. Similar labelling studies with the cell line and its cloned progeny demonstrated the synthesis of a protein identical in size and antigenicity to that synthesized by the marrow cells and found in the patient's serum. These experiments clearly demonstrated that, in this case of heavy chain disease, the deleted protein was the synthetic product of a clone of malignant lymphoid cells.