OBJECTIVE The technology of magnetic field-assisted immuno-agglutination of superparamagnetic particles allows sensitive detection of biomarkers in whole blood. However, we observed non-specific agglutination (NSA), due to interfering plasma proteins, that negatively affects C-reactive protein immunoassay. The objective of the study was to identify the plasma proteins involved and to eliminate these interferences. METHODS Plasma was fractionated by size exclusion HPLC and each fraction was tested for non-specific agglutination. In addition, plasma proteins bound to magnetic particles were analyzed by SDS-gel electrophoresis and identified by mass spectrometry. RESULTS We found that NSA was due to the binding of some lipoproteins to the particles. NSA was observed in the presence of purified LDL and VLDL but not HDL. NSA was mediated by the binding of ApoB100 to magnetic particles through its heparin binding sites. These interferences could be eliminated by addition of heparin or other polyanions like dextran sulfate to the assay buffer. CONCLUSIONS NSA results from the binding of some plasma lipoproteins to magnetic particles. The use of a polyanion to eliminate these interferences allows the formulation of a stable reagent.