Waldenström macroglobulinemia (WM) is a B-cell malignancy that is characterized by the production of a monoclonal IgM protein, a lymphoplasmacytic infiltrate in the bone marrow, and associated symptoms including anemia, lymphadenopathy and hyperviscosity. The aberrant production of a monoclonal IgM in the serum is a major factor causing significant morbidity in patients with this disease, yet little is known about the mechanisms that regulate monoclonal protein synthesis. While recent gene array studies and serum analysis have shown that IL-6 is elevated in WM patients suggesting an important role for this cytokine in this disease, the precise role played by IL-6 in WM is unknown. Using a multiplex ELISA approach to screen sera from WM patients, we confirmed that IL-6 was significantly elevated (p<0.0019) in patients (n=20) compared to controls (n=20). Serum levels of IL-6 in WM patients correlated with elevated levels of β2-microglobulin (p<0.0019). Additionally, we also found that serum levels of CCL5 (Rantes) were significantly elevated in WM patients (p<0.0001). CCL5 has been shown to regulate IL-6 secretion, and we therefore wanted to determine if CCL5 influenced IL-6 expression in WM and what the subsequent consequence of IL-6 stimulation was on WM cells. To define the source of IL-6 in the tumor microenvironment, we used stromal cells from the bone marrow of healthy donors, malignant cells from patients with WM, and the BCWM.1 WM cell line, and tested their ability to secrete IL-6 by ELISA. All cell types secreted IL-6, with stromal cells secreting the most. We then tested the ability of CCL5 to induce IL-6 secretion by WM and stromal cells. CCL5 significantly increased IL-6 secretion by stromal cells (p<0.03) and also increased IL-6 secretion by fresh CD19+ CD138+ cells from WM patients (p<0.02). Using fresh patient WM cells and the BCWM.1 WM cell line as a model, we then determined the effect of IL-6 on growth of WM cells. We found that IL-6 had a modest effect (mean=20% increase, range=5–41%) on cell proliferation (p<0.0039) but had no effect on cell viability. In contrast, when we addressed the role of IL-6 on IgM secretion, we found that IL-6 increased IgM secretion by BCWM.1 cells in a dose-dependent manner. The IL-6 mediated increase in IgM secretion was abolished in the presence of neutralizing antibodies to IL-6. When we analyzed the downstream signaling events activated by IL-6 in WM cells we found that stimulation of BCWM.1 cells, which express the IL-6R, resulted in phosphorylation of Stat1, Stat3 and Erk1/2, but not Akt. Using a mitogen activated protein kinase (MAPK) inhibitor, we could inhibit the IL-6-mediated phosphorylation of Erk1/2. Similarly, using a JAK1 Inhibitor, we could inhibit IL-6 mediated signaling through Stat1 and Stat3. In summary, we have clearly shown that IL-6 significantly upregulates IgM secretion by WM cells and increases their proliferation. We have also demonstrated the ability of both the malignant cells and the stromal cells to secrete IL-6, and that this secretion is regulated in part by CCL5. We have found that WM cells express IL-6R, and that IL-6 induced signaling is through both the MAPK and Jak/Stat pathways. Therapies targeting IL-6 secretion or the IL-6 signaling pathways may therefore provide clinical benefit to patients with WM; not just by inhibiting the malignant cells but by down regulating the production of the monoclonal protein.