It has been reported recently that freshly isolated human myeloma cell cultures proliferate in response to added interleukin-6 (IL-6). Endogenous levels of IL-6 found in the same cultures suggested that an autocrine growth loop may contribute to cell growth. However, the lack of homogenous cell populations in primary myeloma cultures has made it difficult to distinguish between paracrine and autocrine growth mechanisms. To precisely address the autocrine growth issue we have evaluated the growth of the human myeloma cell line, U266. We have found that a neutralizing anti-IL-6 monoclonal antibody can inhibit U266 proliferation. Furthermore, the addition of IL-6 antisense oligonucleotides also inhibits U266 proliferation. These effects are reversed by adding IL-6, suggesting the presence of an autocrine loop. Using bioassays with two different IL-6-dependent cell lines, we were able to detect IL-6 in concentrated U266 supernatants. IL-6 mRNA was detected by polymerase chain reaction amplification of cDNA. Cell cycle parameter analysis shows that IL-6 acts to release a block in G1. Taken together these results present conclusive evidence for IL-6-mediated autocrine growth in the U266 human myeloma cell line.