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CD163 is a monocyte/macrophage restricted transmembrane glycoprotein and a member of the scavenger receptor cysteine-rich (SRCR) family of proteins. SRCR proteins are typically associated with the immune system. The regulation of CD163 by cytokines and glucocorticoids suggests that it plays a role in inflammatory processes. While CD163 is expressed as a membrane-bound protein, it has been shown to be actively shed from the surface of monocytes in a protease-dependent fashion when cells are stimulated with a phorbol ester. To better elucidate the function and biological importance of CD163, we have developed a solid-phase sandwich enzyme linked immunosorbant assay (ELISA) for the detection of soluble CD163 in biological fluids. This assay has good repeatability both within and between runs (coefficients of variation (CVs) of 3.2% and 7.1% or better, respectively). While detection of CD163 was inhibited by ethylenediamine tetraacetic acid (EDTA), CD163 immunoreactivity was not altered by the addition of heparin or hemoglobin. This report details the development of this novel assay for soluble CD163 and provides the first evidence of CD163 immunoreactivity in normal plasma and serum samples.