Interleukin-6 (IL-6) has been shown to stimulate the proliferation of multiple myeloma cells purified to a high degree from human bone marrow. IL-6 production in multiple myeloma has been attributed to cells belonging to the myeloma clone, thus supporting a mechanism of autostimulation. In addition, it has been shown that IL-6 may be produced by auxiliary cell populations of the bone marrow that are not part of the myeloma clone. A definitive separation of both putative sources for IL-6 may be difficult to achieve in fresh patient IL-6 growth requirement and production by pure myeloma cell populations using seven human myeloma cell lines (OCI-My 1 to 7) that were established from patients with advanced disease. The proliferative response of each line to recombinant IL-6 was measured in a clonogenic assay providing human plasma and methylcellulose as a viscous support and by 3H-thymidine uptake in liquid suspension culture. We observed marked heterogeneity, ranging from IL-6-dependent colony formation by OCI-My 4, to IL-6-independent growth. All lines expressed mRNA for the IL-6 receptor. Expression of IL-6 mRNA was analyzed after amplification by polymerase chain reaction and was present in five of seven lines. IL- 6 protein was detected by enzyme-linked immunosorbent assay (ELISA) in the culture supernatants of two lines (OCI-My 3 and 2). Its functional activity was confirmed in a bioassay using the IL-6-dependent murine hybridoma line B 13.29. This activity was neutralized by anti-IL-6 antibody. Two lines did not express mRNA for IL-6. The remaining three lines expressed mRNA for IL-6, but did not secrete IL-6 protein. Immunoprecipitation experiments with lysates of one of these three lines did not detect the presence of IL-6 protein. These results suggest that autocrine stimulation by IL-6 may occur in some cell lines derived from patients with multiple myeloma. However, it does not represent a universal mechanism in myeloma cell growth.