Aqueous extracts of fresh garlic (Allium sativum L.) inhibited efficiently the activity of adenosine deaminase (ADA) of cultivated endothelial cells. The IC50 value (range between 6 and 120 micrograms per ml) depended on the origin and storage time of the fresh garlic. The aqueous extraction of dried garlic powder showed also an inhibition if ADA activity, but the IC50 value was in the range of 2.5 mg per ml indicating that parts of the active principle were lost during the preparation of the garlic powder. The inhibition of endothelial ADA by garlic extracts seems to contribute to the hypotensive activity and vessel protective effects of A. sativum L.