The soluble interleukin-6 receptor (sIL-6R) consists of the extracellular domain of the membrane-bound IL-6 receptor (gp80) found on many types of cells. Contrary to most other soluble cytokine receptors, it possesses in vitro agonistic properties, yet its physiologic role remains unknown. We have generated a cDNA encoding the rat sIL-6R and have expressed and purified the protein using Escherichia coli and baculovirus systems. Analysis of purified protein by electrophoresis and silver staining showed a single band migrating at 35 kDa for E. coli (nonglycosylated) and at 47 kDa for baculovirus-derived material. The purified protein is biologically active, as determined by the ability to convert human hepatoma cells (HepG2) from nonresponsive to responsive to rat IL-6 and induce acute-phase protein synthesis. Most important, we show that rat sIL-6R directly induces proliferation of the IL-6-dependent murine hybridoma cell line (B9) in an IL-6-like manner, with 50% proliferation induced by 100 ng/ml of baculovirus-derived receptor protein. Physiologic concentrations of sIL-6R dramatically enhance the sensitivity of B9 cells to IL-6, indicating that the bioassay for IL-6 is susceptible to modulation by the presence of sIL-6R in rodent serum samples. This sIL-6R-dependent B9 cell proliferation is fully abrogated by antibodies directed against rodent IL-6 and indicates autocrine production of low amounts of IL-6 by the B9 cell line.