In a consecutive series of 65 patients treated by allogeneic bone marrow transplantation, we have compared the efficacy on prevention of bacterial infection of total gut decontamination with oral non absorbable antibiotics or with oral absorbable broad spectrum antibiotics. On day-8, all patients were randomly allocated to one group: Group I received orally pefloxacin 400 mg/day and penicillin 3 M UI/day. Group II received capsules containing cephalothin 250 mg, gentamicin 20 mg and bacitracin 150 UI. Each patient received 9 to 12 capsules per day according to body weight. Patients less than 5 years old or with severe hepatic abnormalities were excluded from the study. All patients were treated in LAF room with usual precautions of asepsis. They received sterile food. In addition, they were on ketoconazole for prophylaxis of fungal infections and acyclovir for prevention of herpes infections. Antibiotics were started on day-8 before bone marrow transplantation and stopped 15 days after discharge from the hospital. 32 patients were allocated to Group I and 33 in Group II. There was no difference between both groups according to age, sex, diagnosis. After transplant, the one year survival and the complications were similar in both groups. The median time of decontamination was 65 days, the mean number of days of agranulocytosis was 20 days, the mean number of days of fever was 7 days in both groups. The compliance was better in Group I but the treatment had to be modified in 9 patients of Group I because of liver abnormalities.(ABSTRACT TRUNCATED AT 250 WORDS)