This paper summarizes the evidence in favor of the theory that many of the platelet’s factors are adsorbed onto its surface from the plasma. It is suggested that this ability of the platelet is one of its basic functions. The platelet adsorbs coagulation and perhaps vascular factors on its surface and carries them through the circulation to the area where they are needed most—at the site of vessel wall injury. This adsorption by the platelet may be an active rather than a passive process, since it requires the expenditure of energy and since it continues only so long as the platelet is viable. In those diseases in which there is marked abnormality of the plasma proteins, there is evidence to suggest that associated coagulation and hemostatic defects may be due to interference with this "atmosphère plasmatique."