Abstract: The invention provides compounds which specifically inhibit factor Xa activity having the structure A1—A2—(A3)m—B, where m is 0 or 1. A compound of the invention is characterized, in part, in that it exhibits a specific inhibition of factor Xa activity with a Ki≦100 &mgr;M, preferably ≦2 nM, and does not substantially inhibit the activity of other proteases involved in the coagulation cascade. The invention further provides methods of specifically inhibiting the activity of factor Xa and of inhibiting blood clotting in vitro and in an individual and methods of detecting factor Xa levels or activity.