Intravenous administration of pefloxacin 400 mg twice daily rapidly decontaminated the bowel from Gram-negative bacilli in ten healthy volunteers. The faecal concentrations of enterococci and yeasts did not change significantly. Further, pefloxacin did not facilitate colonization of the bowel by a highly resistant challenge strain (Klebsiella pneumoniae, MIC = 56 mg/l). The diffusible faecal concentration of pefloxacin was between 110 and 260 mg/l in all samples from day 3 of treatment onwards. It is concluded that parenteral administration of pefloxacin is very effective for decontamination of the bowel from Gram-negative bacilli and provides reliable prophylaxis against colonization of the bowel by highly resistant Gram-negative bacilli ingested with food.