To effectively purify of IL-1 inhibitor from human febrile urine, we have established monoclonal antibody that reacts with human recombinant interleukin l$eta$(IL-1$eta$). The antibody, designated ON-1, was highly specific to IL-1$eta$ and no cross-reaction with other cytokines(IL-l$alpha$ and IL-4) was observed. As the results of ELISA inhibition assay and Western blotting method, it was further identified that ON-1 had high binding specificity with IL-1$eta$. IL-1 receptor binding material from febrile patient urine was effectively purified with affinity column chromatography which conjugated with ON-1. This urinary material inhibited the thymocyte proliferation in a dosedependent manner. IL-l$eta$ induced thymocyte proliferation activity was inhibited to 67.3% at 6 $mu extrm{g}$ of the purified urinary material. The result may suggest that this urinary material the purified urinary material. The result may suggest that this urinary material will have antagonic effect on IL-1 action mechanism and act IL-l$eta$ inhibitor.