Snake venoms have evolved as complex cocktails of pharmacologically active proteins and polypeptides and thus afford excellent natural library of peptide/protein toxins that target mainly neuromuscular and circulatory systems. They bind to key receptors, ion channels and enzymes in with high specificity and exhibit potent pharmacological effects. Thus research on snake venom toxins assist in the development of (a) research tools which help in delineating normal physiological processes at cellular or molecular level; (b) diagnostic tools which help in the identification of specific errors in the physiologic functions of patients; and (c) therapeutic agents which help in the prevention/treatment of specific disorders or diseases. We recently examined several toxins from Bungarus (krait) venoms for their anticoagulant properties. Here I will describe the characterization of a novel anticoagulant protein. This Kunitz-type protease inhibitor selectively inhibits FXIa (henceforth termed Fasxiator: Bungarus fasciatus FXIa inhibitor). Fasxiator exhibits negligible inhibition on other proteases of the blood coagulation cascade. It is the first exogenous inhibitor of human FXIa. Through systematic and selective point mutations, we improved the inhibitory potency (1000 fold) and selectivity towards FXIa and developed a highly potent and selective FXIa inhibitor. The most promising double mutant shows in vivo efficacy in mice carotid artery thrombotic model. Thus our proof-of-principle data support the further development of Fasxiator as a novel anticoagulant candidate.