Monoclonal gammopathies (MG) are monoclonal proliferative disorders of B cells at the differentiation stage of Ig production. They can be detected in the serum, either as transient or as persistent homogenous immunoglobulin (H-Ig) components. The exact phenotype, localization, and cell lineage origin of the precursor cells of MG are unknown, but may be crucial for both the correct diagnosis and for timely efficient treatment of the malignant forms. We used for the first time transgenic (Tg) mice (Sp6; mu/kappa) to study the origin of MG. In the mu, kappa Tg mice a small proportion of B cells can still produce endogenous IgM. These cells are of B-1 cell origin. The MG in Tg mice showed a later onset and a lower frequency than those in littermate control mice, mainly due to a four times lower frequency of benign monoclonal gammopathy. The 10% of B-1 cells that were able to produce endogenous Ig led to the development of MG in a frequency that was half the number of MG found in normal littermates. None of the MG in Tg mice produced an Ig of the Tg origin and therefore it can be concluded that they originated from B-1 cells.