SDS-PAGE demonstrates that antithrombin Cambridge II is predominantly a substrate of thrombin in the presence of heparin. The electrophoretic mobility of plasma alpha (lane 2) and beta (lane 3) antithrombins compared to that of recombinant β-S137A (lane 4) and S137A, A384S (lane5) demonstrates the anticipated size of the recombinant β-antithrombin. An excess of thrombin (lane 15) was reacted with plasma alpha (lanes 6, 9, and 12), recombinant beta (lanes 7, 10, and 13), and the Cambridge II variant (lanes 8, 11, and 14) with and without heparins as indicated (-GAG for no glycosaminoglycan, +Penta for with the pentasaccharide, and +FLH for with full-length heparin). The gel clearly shows that nearly all of the antithrombin reacts with thrombin and either forms complex (high molecular weight bands) or cleaved antithrombin (AT^*). The Cambridge II variant is mostly cleaved in the presence of FLH.