AbstractExposure of heparin‐bearing anticoagulant materials to whole blood, plasma, or serum has been shown to lead to partial elution of heparin. Similar effects are obtained when graphite–benzalkonium–heparin (GBH) or poly–4–vinyl–pyridine–heparin (PVPyrH) surfaces are exposed to bovine plasma protein fractions. Loss of heparin from these anticoagulant materials appears to be dependent on the nature of the cationic surface and on the particular protein fraction used. With whole blood and plasma, and with most of the protein fractions studied, it was found that a larger fraction of the original heparin was eluted from GBH than from PVPyrH. Surfaces exposed to bovine Cohn fraction IV lost considerably more heparin than similar surfaces exposed to Cohn fractions I, II, III, or V, and fraction V removed less than the others, at the same or higher concentrations, and less from PVPyrH than from GBH.