To investigate if oral, nonabsorbable antibiotics prevent bacterial infections in cirrhotics with gastrointestinal hemorrhage, 140 consecutive patients were randomly allocated into two groups: 68 patients (Group I) were given oral, non absorbable antibiotics (gentamicin + vancomycin + nystatin or neomycin + colistin + nystatin) from the inclusion into the trial up to 48 hr after cessation of the hemorrhage, or until emergency surgery or death in those cases who continued bleeding; and 72 patients (Group II) did not receive oral, nonabsorbable antibiotics. Both groups were similar in relation to clinical and laboratory data and characteristics of the hemorrhage. The incidence of infection was significantly lower in Group I than in Group II (11 patients in Group I and 25 in Group II developed proved infections; p less than 0.025). This difference was due to the fact that spontaneous bacteremia and peritonitis and urinary tract infection caused by enteric bacteria occurred almost exclusively in Group II. Two patients of Group I and 10 of Group II developed spontaneous bacteremia and/or peritonitis caused by enteric bacteria (p less than 0.025). These results indicate that prophylactic administration of oral, nonabsorbable antibiotics markedly reduces the incidence of infections caused by enteric bacteria in cirrhotic patients with gastrointestinal hemorrhage.