The substrate behavior of antithrombin toward thrombin in the presence of full-length heparin is dependent on ionic strength. The linear fit of the semilog plot of stoichiometry of thrombin inhibition versus ionic strength demonstrates nearly identical x-intercepts for alpha (circles), recombinant beta (squares), and Cambridge II (triangles) antithrombins. The x-intercept corresponds to the ionic strength where the stoichiometry of inhibition will be one and suggests full inhibitory activity for Cambridge II if thrombin is not bound to heparin.