Determinations were made of thromboplastin activities of the various layers of the human aorta, pulmonary artery, and inferior vena cava. The activity measurements were performed at 30°C by a modification of Astrup's procedure. Thromboplastin values of the vascular samples were expressed in percentages of average activity of the grey matter of human brain tissue, using the double logarithmic plot for the calculation. The mean activities found for the intima, media, and adventitia of normal aortic samples were, respectively, 3.4, 2.8, and 1.8% of human brain tissue (grey matter). Moderately lower values were observed for fibrous-lipid arteriosclerotic tissue portions, and extremely low thromboplastin activities were recorded for ulcerated tissue areas. The layers of the inferior vena cava usually showed activities of the same order of magnitude as those found for the aorta (intima, 3.5; media, 3.0), whereas significantly lower thromboplastin values were recorded for the intima (1.2) and media (1.4) of the pulmonary artery samples.