Three monoclonal antibodies against human interleukin-6 were established and characterized. One antibody was shown to strongly neutralize both the Ig-inducing and hybridoma/plasmacytoma growth activity of interleukin-6. The results of its epitope analysis using protease treated interleukin-6 and immobilized antibody indicated that this neutralizing antibody binds to a peptide corresponding to Leu151-Lys171 of interleukin-6 molecule. Further analysis using synthetic peptides showed that a shorter peptide corresponding to Ala153-Thr162 can also inhibit the binding of the antibody to interleukin-6. These results suggest that this carboxyl-terminal region plays a crucial role in interleukin-6 functions.