Antibiotic excretion into the bile was studied using LMOX and CMZ in 16 postoperative cases of biliary atresia patients who had had hepatic portoenterostomy with Suruga II type enterostomy with the following results: Group I: Excellent excretion which was almost the same as that seen with adult patients; Group IIa: Good or poor excretion, depending on the amount of bile flow and liver function; Group IIb: Very poor excretion; Group III: Good excretion but depending on the amount of bile flow and liver function. As the total bile acid level in bile showed a higher level, the biliary excretion of antibiotics was greater. Our study indicates that antibiotic excretion into the bile in infants is closely related to the condition of the liver function and the biliary passages.