Vitronectin (S protein) and clusterin (SP-40,40/cytolysis inhibitor) are non-homologous, multifunctional proteins which both inhibit complement lysis. Vitronectin is an adhesive protein which binds strongly to polystyrene by hydrophobic interactions. The current study demonstrated that clusterin adsorbed even more efficiently to polystyrene than did vitronectin. This adsorption increased in the presence of Tween 20 and was not abolished by blocking or by the use of other detergents. In double antibody enzyme immunoassays such non-specific binding might invalidate the results. However, the non-specific binding of both proteins was efficiently abolished by the following experimental format: Dynatech Immulon 2 microtiter plate, acidic sample buffer (pH 6.0) containing 0.2% Tween 20 and high sample dilution. Vitronectin was successfully quantitated using this approach, but the measurement of clusterin was not reliable because of high inter-well variation of binding. However, since few serum proteins adsorb to polystyrene in the presence of detergents, clusterin was successfully quantitated in a single antibody enzyme immunoassay in which samples were coated directly onto Nunc Maxisorp plates in the presence of 0.2% Tween 20. In normal blood donors the serum concentration (median and 2.5-97.5 percentile) of vitronectin was 0.34 g/l (0.24-0.53) and of clusterin 0.34 g/l (0.25-0.42).