Excretion of a new semisynthetic analogue of cephalosporin, cefotiam (CTM), and cefazolin (CEZ) in bile was investigated in dogs with experimental obstruction of biliary tract. In the control group, peak concentrations of antibiotics in bile after 50 mg/kg intravenous administration were as follows: In CTM administrated groups, 5,302 micrograms/ml at one hour after administration; and in CEZ administrated group, 1,249 micrograms/ml after one hour. The total biliary excretory rates summed to 6 hours after administration of antibiotics were 7.8% in the CTM group, and 2.1% in the CEZ group. On the other hand, in the biliary tract obstruction group, the peak concentration in bile in CTM group was 65.7 micrograms/ml after 1 hours, and in CEZ group, 62.4 micrograms/ml after 2 hours. The total biliary excretory rate in the CTM group was 0.202%, while in the CEZ group 0.12%. The peak concentration and the excretory rate of CTM in bile was four times higher than that of CEZ in the control group. No significant difference was revealed in the peak concentration and the excretory rate in bile between CTM and CEZ in the biliary tract obstruction group.