We have recently identified a soluble plasma form of CD163 sCD163, the macrophage receptor for clearance of haptoglobin-haemoglobin complexes, and we have observed highly elevated levels of sCD163 in subgroups of haematological patients. In the present study, we describe the optimization and characterization of a sandwich ELISA for the determination of the concentration of sCD163 in plasma and serum. The optimal concentrations of antibodies were determined systematically and the assay was calibrated by CD163 purified from human spleen membranes. The minimum detection limit was below 6.25 microg/L. Recovery of CD163 added to plasma samples was 106%. The concentration of sCD163 in paired serum and plasma samples correlated well (r2=0.99); however, serum levels were 1.1 times higher than the plasma levels. The addition of haptoglobin-haemoglobin complexes did not influence the assay. A very high stability of sCD163 was measured in whole blood and in plasma subjected to different temperatures and after repeated freezing and thawing.