Labeling indices (LI) provide a rapid measure of the bone marrow (BM) plasma cell proliferation rate and are useful in the diagnosis and prognosis of monoclonal gammopathies. Because circulating B cells may be a part of the neoplastic clone, we examined peripheral blood B cells that were producing the same cytoplasmic light chain isotype as the patient's monoclonal; protein (M-protein) and determined the peripheral blood LI (PBLI) by a two-color immunofluorescence bromodeoxyuridine method. The 105 patients studied were divided into three disease activity groups by standard clinical criteria. Median PBLI was 0.2% for the 29 patients with inactive monoclonal gammopathies (monoclonal gammopathy of undetermined significance [MGUS] and smoldering multiple myeloma [SMM]), 0.8% for the 35 patients with new, untreated multiple myeloma (MM), and 1.7% for the 41 patients with relapsed MM. These differences between groups were statistically significant (P less than .001, Wilcoxon). Four patients had high PBLI but clinically inactive gammopathy at the time of study, and all developed active MM within 6 months that required treatment. In 92 patients a BMLI was performed simultaneously with the PBLI (rank correlation coefficient, 0.69). In patients with new, untreated MM, use of both tests identified 72% of patients (23 of 32) with high LI, rather than 56% (18 of 32) by BMLI alone or 63% (20 of 32) by PBLI alone. These results suggest that PB B cells bearing the same cytoplasmic light chain isotype as the monoclonal protein are part of the malignant clone and can be kinetically active. The LI of these cells can provide a measure of disease activity and may help to differentiate active from inactive disease.