A systematic in vitro study has been carried out of the blood clotting properties of rabbit peritoneal leukocytes. Rabbit heterophils have been shown to possess weak but definite tissue thromboplastic activity. They also contain an anticoagulant activity which is active in the intrinsic clotting system, in the extrinsic clotting system with brain thromboplastin, and in clotting systems with Russell’s viper venom. When the thromboplastic activity of the white cell itself initiates clotting, the anticoagulant is much less effective and the procoagulant effect of the WBC suspension predominates. Rabbit heterophils do not bind plasma clotting factors, do not activative factor V, and, under our conditions, do not precipitate fibrinogen from normal plasma, from plasma exposed to traces of thrombin in vitro, or from plasma of rabbits given endotoxin. The relation of these findings to the role of the granulocyte in the pathogenesis of the generalized Shwartzman reaction has been discussed. * Supported by grant HE-6128-05 from the National Heart Institute, IJSPH, by The Norwegian Research Council for Science and the Humanities, and by The Norwegian Council on Cardiovascular Diseases. ** Work done at Medical Department A, Rikshospitalet, Oslo, during sabbatical leave supported by a special fellowship from the National Heart Institute, USPH.