To explore the mechanisms involved in the pathogenesis of human multiple myeloma (MM), we investigated the potential role of interleukin-6 (IL-6), a B-cell differentiation factor in humans, and a growth factor for rat/mouse heterohybridomas and murine plasmacytomas. Using a heterohybridoma assay, we found that two well-documented human myeloma cell lines, RPMI 8226 and U266, did not secrete IL-6 and did not express RNA messengers for IL-6. Neutralizing antibodies to IL-6 did not inhibit their proliferation, and recombinant IL-6 did not stimulate it. Taken together, these data show that IL-6 is not the autocrine growth factor of these human myeloma cell lines. A high production of IL-6 was found in the bone marrows of patients with fulminating MM, compared with patients with inactive or slightly active MM, or to healthy donors. This IL-6 production was assigned to adherent cells of the bone-marrow environment but not to myeloma cells. A spontaneous proliferation of myeloma cells freshly isolated from patients was observed in short-term cultures. Recombinant IL-6 was able to amplify it two- to threefold. The spontaneous proliferation of the myeloma cells was inhibited by anti-IL-6 antibodies and reinduced by recombinant IL-6. After 2 to 3 weeks of culture, the myeloma-cell proliferation progressively declined and no IL-6-dependent myeloma cell lines could be obtained despite repeated additions of fresh IL-6 and costimulation with other cytokines such as tumor necrosis factor (TNF)beta, or IL-1 beta. These data demonstrated a paracrine but not autocrine regulation of the growth and differentiation of myeloma cells by IL-6.