In order to determine the effect of parenteral antibiotherapy on the fecal flora in patients with profound and prolonged granulocytopenia, we initiated a prospective study of 62 cases of autologous bone marrow transplantation following high-dose chemotherapy. All patients were children from 2 to 18 years old, isolated in a protective environment, receiving a diet low in viable microbial content but no oral non-absorbable prophylactic antibiotics to decontaminate the gastrointestinal tract. Bacteriological analysis of fecal flora was conducted at least once a week before and during parenteral antibiotherapy, administered at the first greater than 38 degrees C febrile episode in these granulocytopenic patients (granulocyte count less than 0.5 X 10(9)/l). The 58 evaluable patients fell into three groups with regard to the systemic antibiotherapy: group A (n = 16): moxalactam + mezlocillin; group B (n = 15): moxalactam + tobramycin; and group C (n = 27): cefotaxime plus gentamicin. Fecal flora suppression was observed in 51/58 cases (88%) (group A: 15/16, group B: 13/15, group C: 23/27). It always occurred within 5 days of initiating parenteral antibiotherapy and persisted in 88% of the 51 patients over the whole period of systemic antibiotherapy. During the latter, fecal recolonization was observed in seven cases (12%), always by Enterobacteriaceae sensitive to the prescribed systemic antibiotherapy, never responsible for septicemia. Since parenteral antibiotherapy alone was able to suppress the gastrointestinal tract flora, the effects of this treatment should be considered in all trials of digestive tract decontamination.