Ingestion of coffee was followed after one-half and one hour by shortening of whole blood fibrinolysis time. The amount of coffee was equivalent to a heaped teaspoonsful and a standard brand of instant coffee was tested. The effect was laregly lost when decaffeinated coffee was taken. It appears that the caffeine in the amounts of coffee ordinarily consumed stimulates fibrinolytic activity but 2 of the 12 subjects were resistant to the effect.