The blood fibrinolytic activity was studied in 93 volunteers belonging to 3 different racial groups and related to their body weight adjusted to the ‘weight-for-height’ standard. No difference was found in this activity when overweight, underweight and standard weight British and Asian males were compared. Increased blood fibrinolytic activity was observed in overweight African Negroes as compared to those of standard weight, and decreased blood fibrinolytic activity was discovered in underweight British women when compared to those of standard weight or overweight. It is suggested that the increased fibrinolytic activity in overweight African males may be due to their increased muscular bulk, since muscles are a potent contributor of plasminogen activator to the circulation.