CS-1170 is a new antibiotic, a derivative of cephamycin C. In vitro, 50 strains of E. coli and Klebsiella consisting of gentamicin-sensitive isolated from the blood and gentamicin-resistant strains isolated from the urine were inhibited at concentrations from 0.4 to 12.5 mcg/ml of CS-1170, whereas only 2 strain of Klebsiella isolated from the blood had MIC more than 50 mcg/ml of the antibiotic. Moreover, CS-1170 was significantly more effective than cefazolin and cephalothin against these strains. Ten strains of gentamicin-sensitive Serratia isolated from the blood and the 2 gentamicin-resistant strains were inhibited at concentrations from 3.2 to 50 mcg/ml of CS-1170 and only one strain was resistant to this agent. All tested Serratia were resistant to cefazolin and cephalothin. CS-1170 was not effective against Enterobacter. Three cases of biliary tract infections consisting of 2 cases of cholelithiasis and a case of carcinoma of bile duct were treated with 4 g/day dosage of CS-1170. The remarkable effects were obtained in the two cases with cholelithiasis, whereas a case with the carcinoma was treated not so effectively by administration of CS-1170.