Unfractionated heparin in the extrinsic system has an action on prothrombinase that is insignificant compared to its antithrombin action. In the intrinsic system, unfractionated heparin does have an indirect antiprothrombinase action because its antithrombin activity inhibits the feedback activation of Factor VIII. Most low molecular weight heparins are not different from unfractionated heparin, although their antiprothrombinase action may be slightly higher. Among these, enoxaparin has the highest antiprothrombinase action, due to a relatively high content of very low molecular weight material. In platelet rich plasma, there is an important difference between unfractionated and low molecular weight heparin in that, up to 0.3 U/ml, unfractionated heparin is completely neutralized by activated platelets (300,000 microliters/l) whereas low molecular weight heparins are not. Therefore, unfractionated heparin in platelet rich plasma acts only on the lag phase of thrombin production and not on the amount of thrombin produced. Low molecular weight heparins significantly prolong the lag time and inhibit the thrombin peak in platelet rich plasma.