The biliary excretion of ampicillin was studied in various conditions. Using an isolated perfused rabbit liver preparation, 4.1% of the amount of ampicillin added to the circulating blood (10 mg) was recovered in the bile where the maximum antibiotic activity (mean: 104 +/- microng/ml) was 4 times higher than in the serum (mean: 24.1 +/- 3.4 microng/ml). In man, provided with T-tube drain, the oral or intravenous administration of 500 mg ampicillin resulted in a biliary excretion of 0.10% of the administered dose. The biliary concentrations thus obtained equalled or exceeded for several hours the minimal inhibitory concentration of the organisms included in the antibacterila spectrum of ampicillin justifying thus the use of this antibiotic in the treatment of biliary tract infections. Per-operatory assays showed that in patients with gallbladder disease or with cystic duct obstruction, ampicillin levels in the gallbladder bile were markedly lower than in the common duct bile.