Biliary excretion of cefaclor, a new orally active cephalosporin, was studied in vitro using an isolated rabbit liver preparation perfused for 3 h (n = 5). Under these conditions, bile recovery amounted to 2.3% of the cefaclor dose added to the circulating blood (10 mg). In humans, after oral administration of a 1-gram dose of cefaclor to cholecystec-tomized patients provided with a T tube (n = 10), a mean biliary peak concentration of 7.6 ± 2.4 µg/ml was observed at the 3rd hour. Cumulative biliary excretion amounted to 0.05% of the administered dose. Assays performed on samples collected during chole-cystectomy in 10 patients 1 h after intake of a 1-gram dose of cefaclor showed mean concentrations of 13.7 ± 1.2 µg/ml in serum, 8.1 ± 1.3 µg/ml in common duct bile and 5.9 ± 1.4 µg/ml in gallbladder bile. These results were compared with the data obtained after administration of seven other cephalosporins studied under identical conditions.