Immunoassays were established for the detection of the 55 kDa and 75 kDa tumour necrosis factor receptor (TNFR) fragments present in urine. The immunoassays were based on pairs of monoclonal TNFR antibodies directed against different epitopes of the 55 kDa and 75 kDa TNFRs. The immunoassays were judged to be specific for unoccupied TNFR since the signals were inhibited by adding recombinant human or murine TNF-alpha, and to a lesser extent by rTNF-beta (LT). Other cytokines such as IL-1 beta, IL-2 or rIFN-gamma did not affect the signal. In a preliminary screening it was found that urines from febrile patients contained higher amounts of 55 kDa and 75 kDa TNFR fragments than did urine from non-febrile individuals. The immunoassays could be used to monitor the purification of the two types of TNFR from the same febrile urine. Furthermore, the sensitivity and the speed of the assay could be increased by the use of magnetic beads as a solid support in the assay.