We evaluated the effect of oral ciprofloxacin on neutrophil recovery in 20 consecutive patients undergoing autologous bone marrow transplantation (BMT) for malignant lymphoma and compared the results with a control group of 20 patients receiving co-trimoxazole and folinic acid. Both groups started the prophylactic antibiotic as well as granulocyte-macrophage colony-stimulating factor (GM-CSF) the day after marrow infusion and continued the former until the onset of febrile neutropenia (median duration of treatment 6 days for co-trimoxazole and 7 days for ciprofloxacin). The time of attain an absolute neutrophil count > or = 0.5 x 10(9)/L was significantly shorter in patients receiving ciprofloxacin (16 days vs 22 days; P = 0.006). There was no difference in time to attain a platelet count > or = 20 x 10(9)/L independent of transfusion or in time to the first febrile episode or incidence of bacteremia. We conclude that antibiotic prophylaxis with ciprofloxacin results in more rapid neutrophil recovery than prophylaxis with co-trimoxazole. This may result from a myelosuppressive effect of co-trimoxazole or an enhancement of neutrophil recovery by ciprofloxacin, or both.