Leukemia inhibitory factor (LIF) and oncostatin M (OSM) are found in appreciable concentrations in synovial fluid from patients with rheumatoid arthritis (RA) but not osteoarthritis. Accordingly, both are potential therapeutic targets in inflammatory diseases of the joints. Several LIF antagonists have been developed. They have the capacity to inhibit the biologic activities of not only LIF but also other interleukin-6 (IL-6) subfamily cytokines, including OSM. Both LIF and OSM share the same receptor, which is part of a cytokine receptor super family in which the glycoprotein 130 (gp130) subunit is a common constituent. The aim of this study was to evaluate the antagonistic potentials of two LIF mutants, LIF05 and MH35-BD. Both are mutant forms of human LIF with reduced affinity for gp130 and greater LIF receptor (LIFR) binding affinity. The results, using Ba/F3 cell proliferation assay, acute-phase protein (haptoglobin) induction analysis in HepG2 human hepatoma cells, a porcine cartilage glycosaminoglycan release assessment for proteoglycan degradation, and a collagen release assay, show that these antagonists inhibit relevant LIF, OSM, and other IL-6 subfamily cytokines in vitro albeit with differential potencies and have, therefore, therapeutic potential for treatment of RA and perhaps other diseases.