In 6 healthy adults the effect of essential oil of garlic on platelet aggregation was studied in vitro with an aggreganometer. The blood was collected in a siliconized centrifuge tube containing sodium citrate. The aggregating agents used were ADP, epinephrine and collagen. In each subject aggregation was studied 3 times: (i) initial fasting control; (ii) immediately after (i) but with essential oil of garlic drawn into the syringe together with the sodium citrate; (iii) 5 days after feeding 0.5 mg of essential oil of garlic daily. Addition of essential oil of garlic inhibited in-vitro platelet aggregation induced by ADP, epinephrine or collagen; the effect was dose-related. Oral administration of garlic also decreased platelet aggregation. Thus, garlic seems to inhibit some aspects of thrombus formation.