The nature of the colony-forming cell in the bone marrow of patients with monoclonal gammopathy, as defined in the stem cell assay described by Hamburger and Salmon, has been studied. It could be shown that the colony-forming cells produce immunoglobulins of the same idiotype, heavy chain and light chain, as the monoclonal bone marrow cells in the patient. Data regarding the presence or absence of J chain in the colonies, the failure to observe isotype-switch in the growing colonies, as well as the lack of inhibition of colony formation using antiidiotypic antibodies, strongly suggest that colony formation in vitro reflects proliferation of the clonogenic stem cell in the bone marrow without apparent differentiation. The stem cell may be of plasma cell nature.

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