An antibiotic drug of aminoglycoside group, gentamicin (GM) for parenteral use was used to 14 hospitalized patients; 5 with acute or subacute cholecystitis, 6 with acute peritonitis (4 cases were due to acute appendicitis, a case was torsion of right ovarian cyst and a case was cecal CROHN's disease), 1 with fistula ani and abscess, and 2 with localized peritonitis after gastrectomy due to gastric ulcer. GM in a dose of 60 mg were administered by intravenous drip infusion for 1 to 2 hours, twice a day for 4 to 12 days. To the cases of biliary tract infection, GM was treated for preoperative chemotherapy and to the other cases GM was treated for postoperative chemotherapy. Clinical response was excellent in 7 cases, good in 6 cases, fair in 1 case and poor in none. No adverse effect was observed. The organisms were isolated in 7 cases, 7 were Escherichia coli, 2 were Klebsiella pneumoniae and 3 were Bacteroides fragilis. The MICs for GM were 0.78--1.56 micrograms/ml in 10(8) and 10(6) cells/ml, except B. fragilis. Before the operation of above cases, GM in a dose of 60 mg (a case was 40 mg) were administered by intravenous drip infusion for 1 to 2 hours in 7 cases (3 biliary tract infection, 2 acute peritonitis and 2 gastric ulcer) and 7 cases by intramuscularly. The materials of common duct bile, gall bladder bile, gall bladder wall, the appendix and other tissues, ascites and serum samples were taken during the operation. GM concentration was measured by bioassay method with Bacillus subtilis ATCC 6633 as test organism. GM concentrations in bile and gall bladder wall after intravenous drip infusion were higher than those after intramuscular administration. In the appendicitis with localized peritonitis, GM concentration in the appendix wall with catarrhal appendicitis was 0.90 microgram/g after intramuscular administration. In the cases with diffuse peritonitis and catarrhal appendicitis, GM concentrations in appendixes were 1.18 micrograms/g and 1.37 micrograms/g after intravenous drip infusion. Therefore, it was supposed that GM could be used safety and usefully by intravenous drip infusion than that by intramuscular administration.