A chemically synthesized heparin pentasaccharide (Institut Choay, Paris, France) has been shown to exhibit an antithrombotic action in a rabbit stasis induced thrombosis model, in an IV dose range of 25 to 200 micrograms/kg (0.5 to 3.5 micrograms/ml plasma circulating concentrations). Ex vivo plasma analysis from treated animals revealed expected anti-factor Xa activity but no direct inhibitory effect against thrombin. Global anticoagulant activities were not found by PT and APTT methods. Platelet activation remained unaffected at the antithrombotic dosages of pentasaccharide. To more specifically elucidate the anti-factor Xa mediated antithrombotic mechanism of action of this pentasaccharide, it was studied in several thrombin generation assays. Pentasaccharide added to human and rabbit plasmas in vitro from 0 to 5.0 micrograms/ml produced a concentration dependent effect up to a 35 to 50% inhibition of generated thrombin. In ex vivo studies similar concentration dependent inhibition of thrombin generation was observed. Analysis of plasma obtained from animals in which a complete antithrombotic effect was observed in vivo demonstrated an approximate 45 to 55% inhibition of thrombin generation. These results indicate that a relationship exists between the pentasaccharide induced inhibition of experimental venous stasis thrombosis and the inhibition of thrombin generation.