We analyzed the effect of in vivo ciprofloxacin and ceftazidime treatment on the development of myeloid progenitors and on the survival of lethally irradiated mice rescued with syngeneic bone marrow transplantation (BMT). Ciprofloxacin treatment (15 mg/kg per dose three times daily for 5 days) enhanced myeloid progenitor (colony-forming cell [CFU-C]) number in the bone marrow and the survival of mice transplanted with suboptimal doses (1 x 10(5) of s[Ngeneic bone marrow cells (BMC). Twenty days postirradiation, 50% (38 of 76) of saline-treated mice transplanted with 1 x 10(5) cells died compared with 25% (19 of 76) of ciprofloxacin-treated mice (p < 0.05). Similarly, ciprofloxacin treatment enhanced survival of mice transplanted with 1 x 10(6) syngeneic bone marrow cells: 50% (38 of 76) of saline-treated mice died within 20 days vs. 15% (12 of 80) of ciprofloxacin-treated mice. In contrast, treatment with ceftazidime did not affect progenitor cell number or survival. On day 8 postirradiation, although lethally irradiated mice transplanted with 1 x 10(5) BMC treated with ciprofloxacin demonstrated similar white blood cell (WBC) and red blood cell (RBC) counts as saline-treated mice, a (1.9 +/- 0.2)-fold increase in the percentage of polymorphonuclear cells (PMN) was observed in the peripheral blood of ciprofloxacin-treated mice. On day 5 postirradiation, ciprofloxacin-treated mice showed a (1.6 +/- 0.2)-fold increase in the number of peritoneal PMN and a 6.5-fold increase in their antibacterial activity towards Salmonella typhimurium in comparison with saline-treated mice.