The saliva of the tsetse, Glossina morsitans morsitans Westwood, has antithrombin anticoagulant activity and inhibits thrombin's esterolytic activity. It has no other detectable anticoagulant properties. The anticoagulant elutes in a single peak on Sephadex fraction, is immediately acting, heat and storage stable, and has a molecular weight of 11-13,000. Unlike heparin it is not neutralized by protamine sulphate or toluidine blue and does not require the co-factor, antithrombin III, for optimal anticoagulant activity. It has similar properties to hirudin, but does not elute with a protein peak upon Sephadex fractionation and has a slightly different molecular weight. Salivary gland homogenates contained neither a plasminogen activator nor fibrinolytic activity. The sera of rabbits used to maintain tsetses, which contained precipitating antibodies against saliva, did not neutralize the salivary anticoagulant in vitro. The properties of this anticoagulant suggest that it might be a potentially useful antithrombotic agent in man.