Median concentrations (±standard errors of the median) of anaerobic bacterial CFU per gram of fecal sample in patients treated with 500 mg metronidazole PO tid and 750 mg ciprofloxacin PO bid (□; n = 68) or with 750 mg ciprofloxacin PO bid alone (▩; n = 66) in the week before and during 5 weeks after allogeneic marrow transplantation. The CFU detection threshold is 10³ per gram of sample. Median concentrations of anaerobic bacterial CFU in patients treated with metronidazole and ciprofloxacin were below the detection threshold between 3 and 5 weeks posttransplant. Differences between both patient groups were compared using repeated measures analysis of variance.22