Interleukin-6 (IL-6) is an important growth and survival factor for myeloma cells. However, the identity of the cells producing IL-6 in vivo remains unclear. Myeloma cells are found closely associated with sites of active bone turnover, and cells of the osteogenic lineage, including bone marrow osteoprogenitors, osteoblasts and bone lining cells, may therefore be ideally placed to synthesize IL-6. We have examined the possibility that human osteogenic cells may produce IL-6 in response to stimulation by myeloma cells. Primary human osteoblasts (hOBs) were isolated from normal donors, co-cultured with the human myeloma cell lines, JJN-3, RPMI-8226 and NCI-H929, and the amount of IL-6 released was determined by enzyme-linked immunosorbent assay (ELISA). All myeloma cells stimulated a significant increase in the production of IL-6 when cultured with hOBs (P < 0.05). Prior fixation of hOBs completely abrogated release of IL-6 in the co-cultures. In contrast, fixed myeloma cells retained the ability to induce IL-6 production, suggesting that hOBs were the principal source of IL-6. Physical separation of myeloma cells from hOBs using transwell inserts caused a partial inhibition of IL-6 release (P < 0.05), whereas the addition of media conditioned by myeloma cells to cultures of hOBs stimulated a significant increase in IL-6 production (P < 0.05). hOBs secreted greater amounts of IL-6 than human bone marrow stromal cells (hBMSCs) (2.2- to 3.5-fold, P < 0.05), but incubating hBMSCs with dexamethasone to stimulate osteoblastic differentiation resulted in an increase in their ability to produce IL-6 (1.7- to 4. 8-fold, P < 0.05) and to respond to myeloma cells (P < 0.05). These data clearly indicate that cells of the osteoblast lineage release significant amounts of IL-6 in response to stimulation by myeloma cells and may contribute to the IL-6 that promotes the proliferation and survival of myeloma cells in vivo.