Two hours before surgery and every 12 hours thereafter 5,000 units of heparin sodium was administered subcutaneously to 100 general surgical patients. Hemostasis was evaluated by a template bleeding time and an activated partial thromboplastin time (PTT). The latter was sensitive to 0.05 units/ml of heparin and gave a straight-line response up to 0.2 units/ml. In the great majority of patients, only a modest elevation of the PTT occurred two and four hours after heparin therapy. However, in 10% to 15% the PTT was prolonged two times or more and in a similar number, PTT after surgery was shorter than baseline values despite heparin. No correlation between PTT prolongation and weight, ponderal index, age, or sex was found. Significant bleeding occurrred in three patients, two from the group of hyperresponders to heparin. Recent aspirin ingestion was implicated in one patient and our evidence indicates that low-dose heparin potentiates aspirin-induced prolongation of bleeding time in certain individuals. Local hematoma formation and discomfort from the injections was not a problem.