Subjects numbered 41, of whom 17 were controls, either patients or staff, and 24 were overweight, 14 grossly so, with mean weight 250 lb. In samples of venous blood plasma fibrinolytic activity, plasma fibrinogen and plasma plasminogen were estimated. There was an inverse relation between fibrinolysis and the ratio observed: standard weight. Activity for normal subjects was, on average, 4.7 units, 2.9 units for moderately and 1.3 units for grossly obese. Differences were significant. Plasminogen levels were similar in all groups. Fibrinogen levels were higher in the grossly obese. Restriction of diet sufficient to induce rapid loss of weight raised the fibrinolytic activity in both lean and obese subjects. In 4 obese subjects fibrinolysis was less enhanced by exertion than in 3 normal subjects.-JE Hammant.