Fifteen patients received amoxycillin/clavulanic acid combination intravenously at anaesthetic induction as part of a three-dose schedule of antibiotic prophylaxis for vascular surgery. After 30 min, the mean serum concentration of amoxycillin was 39.9 mg/l and clavulanic acid was 9.3 mg/1. Mean amoxycillin tissue concentrations in fat, 5.1 mg/l; vein, 7.5 mg/l; and artery, 4.8 mg/l, were lower than serum values. Mean clavulanic acid concentrations were: Fat, 0.7 mg/l; vein, 1.2 mg/l; and artery, 1.6 mg/l. At the end of surgery, the serum amoxycillin level was always > 6 mg/l and clavulanic acid > 2 mg/l, which were both still within the therapeutic range. Concentrations in fat had declined by the end of the surgery ranging from 0.76 to 5.2 mg/l for amoxycillin and from 0.03 to 0.6 mg/l for clavulanic acid, there being almost none detected in four cases. Serum and tissue concentrations of both compounds 30 min after injection were sufficient to inhibit the growth of 97% of pathogenic organisms recovered from the skin of 135 vascular surgical patients. In a few cases tissue levels of the compounds were below inhibitory concentrations for conforms and anaerobes at the time of skin closure, although they were still effective against most Gram-positive organisms. Amoxycillin/clavulanic acid combination is suitable antibiotic prophylaxis for vascular surgery.