Multiple myeloma (MM) is characterized by the expansion of terminally differentiated plasma cells. It is still uncertain whether the clonogenic fraction is confined to the plasma cell or pre-plasma cell compartment. We examined the immunoglobulin (Ig) rearrangement of myeloma heavily infiltrated bone marrow cells with a probe from the heavy chain J region (JH) and the BamHI, EcoRI and HindIII restriction enzymes which are appropriate for the detection of clonal VDJ recombination. In 23/39 MMs, clonal Ig gene rearrangement was detected with BamHI, EcoRI and HindIII enzymes. Unexpectedly, in 14/39 patients both BamHI and EcoRI failed to detect Ig rearrangement, whereas HindIII consistently demonstrated VDJ recombination. The 5' sites of BamHI, EcoRI and HindIII restriction fragments are precisely defined by the VDJ rearrangement. Since the 3' ends of BamHI and EcoRI restriction fragments are downstream from the switch mu region and change in size during switch recombination, the absence of rearranged bands is determined by several autonomous recombinations affecting the switch region. By contrast, the 3' ends of HindIII restriction fragments are upstream, their size does not vary during isotype switch allowing the constant detection of clonality. Accordingly, in 35% of patients the clonogenic fraction seems to originate from a pre-switch B cell. This B cell will differentiate to a mature plasma cell developing multiple independent switch recombinations, as the variable mechanism of switch recombination suggests.