We describe a microtechnology for the study of the coagulation system in newborn infants. Interpretation of results demands an understanding of the techniques used and the nature of the control population from which normal values are drawn. We have examined two syndromes which represent the majority of hemostatic disorders of sick newborn infants. The first is thrombocytopenia resulting from bacterial infections in which there are minimal changes in the levels of blood coagulation factors and little tendency to bleed. The second is a syndrome of disseminated intravascular coagulation in which there is a profound disturbance in the coagulation mechanism, relatively little change in platelet counts, a severe hemorrhagic diathesis, and widespread ischemic necrosis.