Twelve infants having undergone extrahepatic cholangiojejunostomy (SURUGA'S procedure II) after portal hepaticojejunostomy as a radical operation for congenital biliary atresia received prophylactic cefotiam (CTM) against biliary tract infection, 50 mg/kg by intravenous drip in 1 hour, followed by serial determinations of bile and serum CTM levels. The patients were evaluated as to degree of CTM excretion into the bile, by classification into 4 groups according to success or failure in establishing the anastomosis, bile outflow and persistence of jaundice. Patients with an established anastomosis, satisfactory bile flow and no longer jaundice showed high biliary antibiotic concentrations whereas lower biliary antibiotic levels were seen in those with failure in anastomosis, poor bile flow and persistent jaundice. In both cases, nevertheless, the antibiotic concentration in bile sufficiently exceeded the MIC80 values of CTM against principal bacterial pathogens causative of biliary tract infections, thus demonstrating clinical significance of the use of CTM for this purpose. The antibiotic excretion into the bile improved with normalization of liver function in an infant displaying satisfactory bile outflow from an early postoperative stage. The bile and serum CTM assay data obtained by the agar well method showed a high degree of correlation with those by high performance liquid chromatography.