Temocillin is a new semisynthetic beta-lactam antibiotic with a spectrum of activity against most aerobic Gram-negative bacteria. We investigated the biliary excretion of temocillin after administration of 1g either intravenously or intramuscularly to 15 patients with normal liver and renal function who were undergoing elective cholecystectomy for gallstones. Very high concentrations of temocillin (up to 8 times the corresponding serum concentrations) were found in common bile duct bile and gallbladder bile of functioning gallbladders 1 to 2 hours after administration. However, no temocillin was detected in the bile of non-functioning gallbladders. Concentrations of temocillin were significantly higher in common bile duct bile (p less than 0.01) and gallbladder bile (p less than 0.05) following intravenous administration, and there was also a significant correlation (p less than 0.05) between common bile duct bile and corresponding serum concentrations for both routes of administration. Temocillin 1g readily achieved therapeutic concentrations in bile, and may prove a useful agent in the prophylaxis and management of Gram-negative biliary infections.