Human sIL-1R II immunoassay is a 3.5 - 4.5 hour solid phase ELISA designed to measure human sIL-1R II in cell culture supernates, serum, and plasma. It contains recombinant human sIL-1R II and antibodies raised against this protein. It has been shown to accurately quantitie recombinant human sIL-1R II. Results obtained with naturally occurring sIL-1R II samples showed linear curves that were parallel to the standard curves obtained using the kit standards. These results indicate that the Immunoassay kit can be used to determine relative mass values for natural human sIL-1R II.