The secretion of the salivary glands of ticks, Ixodes ricinus, contains anticoagulant substances. Some years ago an antithromboplastin, ixodin, was described. The present paper refers the isolation and characterization of a second anticoagulant substance of the ticks named ixin. Ixin proved to be a relatively stable and specific thrombin inhibitor. The multi step procedure for isolation results in a 850-fold purification. The preparation obtained has a specific activity of 250 antithrombin units/mg. It is not homogeneous and still contaminated. The substance was assumed to be a miniprotein.