Interleukin 1(IL-1) is a cytokine that acts as a mediator of various immune responses and inflammation. IL-1 binds to its receptor molecules expressed on the responding cells to mediate these responses. Proteins with IL-1 inhibitory activity from febrile patient urine block the action of IL-1 by binding to IL-1 receptors with similar affinity IL -1. They show no IL-1 like action. We prepared FITC-Labelled IL-1 and established a receptor binding assay using flow cytometer. Mouse thymocytes expressing IL-1 receptor molecules on their surface were employed. In this binding, we obtained proportional binding of IL-1 to the quantity of IL-1 in the range of 2.5 to 100 units with a constant receptor concentration, and the number of receptor, in terms of thymocyte number, could be varied from 10^6 to 10^7. This assay peared to be more rapid, simple, and convenient that the conventional binding assay using ^(125)I-labelled IL-1. The analysis of IL-1 inhibitors from febrile patient urine by the binding assay showed good correlation with the result obtained by cell proliferation method.