The mononuclear cells in the peripheral blood are implicated in the myeloma process especially with the presence of peripheral blood plasma cells (PBPC) and clonal B lymphocytes found using phenotypic or gene rearrangement techniques. The purpose of this study was to look for aneuploidy in the two main B cell components of the peripheral blood: PBPC and CD20-positive B lymphocytes. Conventional cytogenetics (CC) or DNA content analysis and fluorescence in situ hybridization (FISH) with centromeric probes were performed on bone marrow plasma cells (BMPC) of 21 patients with multiple myeloma and peripheral blood cells were studied as follows: immunostaining to look for PBPC and to assess their number, image analysis cytometry for the determination of their DNA content, and FISH chromosomes analysis. FISH was performed using probes against the chromsomes that were lost or gained in BMPC and was coupled with immunostaining of the relevant light chain or CD20 antigen to study PBPC or B lymphocytes, respectively. Monotypic PBPC were found in 16 patients. Their DNA content was the same or nearly the same as for BMPC and they exhibited the same monosomies or trisomies as those found within their BM counterpart. By contrast, DNA content of mononuclear cells other than PBPC was within normal ranges, and in 13 of 15 patients CD20-positive B lymphocytes failed to show chromosomal changes by FISH analysis. In two patients however, a few CD20+ cells with lymphoid morphology exhibited chromosome changes, hypothesizing that a few cytogenetically abnormal B cells without plasmocytic morphology may circulate. From these data, we conclude that PBPC share the same genetic abnormalities as BMPC and thus belong to the malignant clone, whereas most peripheral blood B lymphocytes are unrelated to the tumor clone.