Stoichiometries of factor Xa and thrombin inhibition determined from residual protease activities with increasing antithrombin concentrations. The stoichiometries of inhibition of factor Xa (A) and thrombin (B) are taken as the x-intercept of the linear regression when residual protease activity is plotted against the ratio of antithrombin to protease. A value of one is normal for serpins and is found for the plasma alpha antithrombin (circles), recombinant beta antithrombin (squares), and recombinant antithrombin Cambridge II (triangles) in the absence of full-length heparin (open symbols). For all antithrombins there is an increase in stoichiometry in the presence of heparin (closed symbols), but this is exaggerated for the Cambridge II variant, which is predominantly a substrate of both factor Xa and thrombin.