The urine of febrile patients has been found to contain high concentrations of an inhibitor of interleukin 1 (IL-1)-induced thymocyte proliferation. The inhibitor is specific for IL-1 and does not block the effects of interleukin 2 (IL-2) or phytohemagglutin (PHA) on thymocytes, and it is not nonspecifically toxic for these cells. IL-1 inhibitor can be found in the urine of normal individuals and afebrile patients, but is present in increased concentrations in the urine of patients with fever of diverse etiologies. Preliminary physicochemical characterization indicates that the inhibitor is a 20-40-kdalton protein.

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