Orgaran is a LMW heparinoid composed of heparan sulphate (83% w/w) of which 4-5% has high affinity for antithrombin, dermatan sulphate (12% w/w) and chondroitin sulphate (5% w/w). To examine the contribution of the low-affinity fraction to Orgaran's antithrombotic activity we have quantitated the binding of plasma proteins to Orgaran and its component fractions in whole, hirudin-anticoagulated human plasma. Antithrombin, largely bound to the high-affinity fraction, and histidine-rich glycoprotein, interacting with low-affinity components, were the dominant proteins bound to Orgaran. Vitronectin, fibrinogen, fibronectin, heparin cofactor II, and apolipoprotein B were also detected in small amounts. The ratio of bound antithrombin, histidine-rich glycoprotein and vitronectin to GAG was negatively correlated with the Orgaran concentration in plasma, implying that the efficacy of Orgaran may not be linearly related to dose. Binding of antithrombin to the high-affinity fraction was not decreased by other plasma proteins or affected by addition of low-affinity material. Moreover, the antithrombin and anti-factor Xa activities of the high-affinity material were unaltered by low-affinity GAGs. On the basis of our results we conclude that the low-affinity material does not contribute to the antithrombotic activity of Orgaran by binding non-anticoagulant plasma proteins and releasing the high-affinity chains to interact with antithrombin and its target proteinases.