A novel human myeloma cell line, OH-2, was established from pleural fluid of a myeloma patient in end stage of the disease. Effects of cytokines on proliferation were analyzed by measuring uptake of 3H-thymidine. Cell surface antigens were detected by flow cytometry. The cell line is dependent on IL-6 for growth and proliferates in response to TNF. There is synergy between the stimulatory response of TNF and IL-6. The cells express both the p55 and p75 TNF receptors. Neutralizing anti-IL-6 did not inhibit TNF-mediated proliferation, showing that TNF acts through a pathway that is independent of IL-6. TNF was more potent than IL-6 in stimulating the growth of primary myeloma cultures (> 99% pure) from the same patient (OH-2-PC), indicating that TNF in selected myeloma patients has a growth-promoting effect equal to IL-6. OH-2 cells produce and secrete monoclonal IgG-kappa.