The fact that the tiny blood platelets play a key role in thrombo sis dates back to observations by Bizzozero (1) and Schimmelbusch (2). They contribute to various aspects of hemostasis, including : (a) forma tion of an adherent cellular plug, buttressed by the fibrin clot; (b) libe ration of vasoconstructor factor(s), e.g. serotonin; (c) participation in the reactions of blood coagulation, partly by providing certain factors from their own substance, and partly by a role of factors in their «plasmatic atmosphere» (3), which is the special topic of this thesis, and, finally (d) by playing a role in clot-retraction, which can provide additional he mostatic function through the approximation of thin vessel walls (4).The fact that in vitro blood clotting normally necessitates the presence of platelets is indicated by the occasionally (e.g. ( 5)) successful experiment of preparing a blood plasma so free from platelets and plate let fragments that it will not clot on simple recalcification even in glass, as well as in silicone. Clotting is restored on adding back the platelets, or certain phospholipids (e.g. brain cephalin), or some enzymic bypass mechanisms (5). Platelets and lipoids, in relation to blood coagulation and the hemorrhagic diseases, with an historical account and critical presentation of investigative methods, were considered in a 1960 mono graph (6), and a didactic review, including many recent conclusions from the extensive modern literature, was published in 1965 (7). The modem work is expounded by most of the leading authorities in a number of fairly recent international and national symposia, e.g. ( 8, 9, 10, 11). Re cent biochemical, morphologic, and clinical research on the physiology of the blood platelets, with excellent citations of the literature, are pre sented by Marcus and Zucker (12). A 1961 review of «the platelet as a sponge» (13) indicates the recent revival of interest in the platelet plas matic atmosphere, as pursued by Roskam and colleagues, and in other laboratories (e. g. (14). Coagulation research, from very many aspects, has long been pursued in our own laboratories. With the advent of visiting research scientists, Dr. Sotirios G. Iatridis (1959 -1963) and the present author (Dr. Panayotis G. Iatridis (1963 -1966)), extensive investigations have particularly dealt with the trigger clotting mechanisms in vitro and in vivo, experimental thrombosis (in dogs and rabbits), cf. (15), the roles of blood platelets, with particular reference to factors in their plas matic atmosphere, and certain enzymes and inhibitors of platelet and other origins. It is the significant research of Dr. P. G. Iatridis which is presented in this monograph, with a critical review of the relevant li terature and comparisons of, and conclusions from, currently available results. The new data add to our basic knowledge in these fields and pave the way for rational future applications in clinical medicine.