Laboratory and clinical comparative investigations with cefotiam (CTM) and cefazolin (CEZ) were performed to confirm efficacy and safety in surgical, biliary tract infections. The following results were obtained. 1) The MICs of CTM against organisms, 20 strains, which were isolated from bile of patients with cholecystitis, were studied, especially those of CTM against E. coli and Klebsiella were 0.2 - greater than or equal to 100 micrograms/ml and 0.1--12.5 micrograms/ml, respectively, considerably lower than those of CEZ. And moreover against CEZ-resistant Proteus morganii and Serratia marcescens, CTM showed potent activities, that is, MIC values, 12.5--50 micrograms/ml and 0.78 microgram/ml, respectively. 2) In cholecystectomized patients, 2 grams of CTM was injected intravenously, followed by determination of bile concentration in gallbladder, common bile duct and concentration of gallbladder and liver tissue about 2 hours after administration. The mean bile concentrations of CTM in gallbladder and common bile duct were 1,213.2 micrograms/ml and 1,287.8 micrograms/ml, respectively. The peak concentration of CTM was 2,919.0 micrograms/ml. The mean concentrations of CTM in gallbladder and liver tissue were 28.5 micrograms/g and 45.7 micrograms/g, respectively. On the other hand, the mean bile concentrations of CEZ in gallbladder and common bile duct were 138.7 micrograms/ml and 128.8 micrograms/ml, respectively. 3) Bile concentrations of CTM was compared with those of CEZ by crossover method. The concentration of CTM was 37.7 micrograms/ml even at 5 or 6 hours after 2 grams intravenously administration. CTM showed extremely higher concentration than CEZ in bile. 4) The clinical effect was studied in 6 cases of surgical, biliary tract infections. The results were excellent in 2 cases, good in 3 cases and poor in 1 case, and the clinical efficacy was 83%. 5) CTM was administered to 6 patients who showed negative to intracutaneous reaction test. Nausea, itching, and eruption were observed in each 1 case after intravenously administration of CTM 2 g, however these adverse reactions disappeared within several hours. Throughout the course of treatment, any unusual laboratory findings related to CTM were not observed.