The cytokine-induced mobilization of progenitor cells is decreased by treatment with ciprofloxacin and polymyxin B. Mice were treated with 100 mg/L ciprofloxacin, 70 mg/L polymyxin B, and 20 mg/L saccharose added to their drinking water for 1 week. While the antibiotic treatment was continued, mice were mobilized with either a single intraperitoneal injection of 30 μg IL-8 (n = 8), or 5 μg G-CSF (n = 15) or 10 μgFL(n = 9), both for 5 days. At 20 minutes after the IL-8 injection and 24 hours after the last injection with G-CSF or Flt 3 ligand, mice were killed and blood was harvested. Results are expressed as means ± SD. *P < .05 compared with the saline pretreated controls (n = 3-10). Results are expressed as means ± SD.