We have a previously reported that interleukin-10 (IL-10) is a potent but IL-6-unrelated growth factor for freshly explanted myeloma cells (Lu et al, Blood 85:2521, 1995). We have also shown that exogenous IL-10 supported the growth of XG-1 and XG-2 human myeloma cell lines (HMCL) through an IL-6-independent mechanism. (Lu et al, Blood 85:2521, 1995). Because the IL-10 receptor does not involve the gp 130 IL-6 transducer, we have attempted to elucidate the mechanisms of IL-10 action on myeloma cells. Our results indicate that the myeloma cell growth factor activity of IL-10 was abrogated by an antibody to the gp 130 IL-6 transducer, indicating that it was mediated through one of the gp 130-activating cytokines. We found that myeloma cells from XG-1 and XG-2 HMCL and from 5 of 6 patients' tumoral samples produced oncostatin M (OM) constitutively but failed to produce IL-6, IL-11 and leukemia-inhibitory factor (LIF). The autocrine OM was inactive in the absence of IL-10 due to lack of a functional OM receptor on myeloma cells. IL-10, by inducing the receptor for LIF (LIFR), produced a functional autocrine OM loop in XG-1 and XG-2 cells and in primary myeloma cells from 2 patients. We also found that some myeloma cell lines (XG-4, XG-6, and XG-7) an fresh myeloma cells from 3 of 6 patients produced an autocrine IL-10 and that these cells constitutively expressed LIFR. One HMCL (XG-7) produced IL-10, OM, and IL-6 an expressed LIFR. The XG-7 cells used OM and IL-6 as autocrine growth factors. We have previously shown that IL-10 could induce IL-11 receptor in myeloma cells and confer on them sensitivity to IL-11 (Lu et al, FEBS Lett 377:515, 1995). Taken together, these results show that IL-10 is a key cytokine for inducing the expression of LIFR and IL-11R and possibly another uncharacterized OM coreceptor on myeloma cells and that OM and IL-10 might be produced by myeloma cells. They also emphasize that all myeloma cell growth factors reported to data involve an activation of the gp130 IL-6 transducer.