The pleiotropic cytokine interleukin-6 (IL-6) interacts with the specific ligand binding subunit (IL-6R alpha) of the IL-6 receptor, and this complex associates with the signal-transducing subunit gp130 (IL-6R beta). Human IL-6 acts on human and murine cells, whereas murine IL-6 is only active on murine cells. The construction of a set of chimeric human/murine IL-6 proteins has recently allowed us to define a region (residues 43-55) within the human IL-6 protein, which is important for the interaction with gp130. Subdividing this region shows that mainly residues 50-55 of the human IL-6 are necessary for this interaction. Recently, another human IL-6 double mutant (Q159E and T162P) showed reduced affinity to gp130 but residual activity on the human myeloma cell line XG-1. Into this IL-6 mutant we introduced the murine residues 43-49 or 50-55 together with two point mutations, F170L and S176A, which had been reported to increase the affinity of IL-6 to the IL-6R alpha. The resulting IL-6 molecule, which contained the murine residues 50-55, was inactive on human myeloma cells and in addition completely inhibited wild type IL-6 activity on these cells. Such an antagonist may be used as a specific inhibitor of IL-6 activity in vivo.