The effects of tumor necrosis factor alpha (TNF-alpha) and interleukin-6 (IL-6) on the cell growth of the myeloma cell line KHM-1A were studied. TNF-alpha strongly induced KHM-1A proliferation, while IL-6 weakly enhanced growth only at low cell densities. When TNF-alpha was combined with IL-6, TNF-alpha-induced growth enhancement was reduced. According to Northern blotting analysis, the m-RNA of both TNF-alpha and IL-6 were detected in KHM-1A. Moreover, monoclonal antibody capable of neutralizing the cytotoxic activity of TNF-alpha inhibited the proliferation of this cell line. These findings suggest that this cell line operates under an autocrine growth mechanism with respect to these two cytokines, IL-6 and TNF-alpha.