The number of circulating plasma cells (CPC) was determined on mononuclear cell preparations after Giemsa (morphology) and light chain staining (immunocytochemistry). Both methods gave reproducible and identical results when CPC were 1% or more. Using this limit, no CPC were observed in MGUS (0/11) and primary amyloidosis (0/2), whereas 45/98 (45.9%) multiple myeloma (MM) pts had > or = 1% CPC. 3/14 pts (21.4%) in stage I, 5/13 pts (38.5%) in stage II, 20/28 pts (71.4%) in stage III, 4/25 pts (16%) at plateau phase, and 13/18 pts (72.2%) at relapse had > or = 1% CPC (p < 0.001). Mean beta-2 microglobulin was 3.77 mg/l and 6.08 mg/l for pts without or with > or = 1% CPC, respectively p = 0.0001). Presence of CPC was also correlated with an higher percentage of bone marrow PC, but not with the number of Ki-67 positive BM-PC, and not with CRP or LDH levels. K/L and Gamma/Alpha CPC isotype ratio showed these cells as monotypic in nearly all pts. The prognostic value could not really be assessed in this study, as only the initial response to therapy was investigated, and the latter failed to give any difference between pts with and without CPC. So, presence of CPC is not an infrequent finding, but is highly related to tumor mass and active disease; in most if not all patients they are monotypic and certainly belong to the malignant clone. Their prognostic value is unclear but under current investigation; CPC are correlated with B-2M level.