The Quantikine Mouse/Rat IL-33 immunoassay is a 4.5 hour solid-phase ELISA designed to measure IL-33 in cell culture supernates, serum, and plasma. It contains E. coli-expressed recombinant mouse mature IL-33 and antibodies raised against the recombinant factor. This immunoassay has been shown to accuray quantitate both recombinant mouse and rat IL-33. Results obtained using natural IL-33 showed dose response curves that were parallel to the standard curves obtained using the Quantikine kit standards. These results indicate that this kit can be used to determine relative mass values for natural IL-33.