The effects of altering the timing of recombinant human granulocyte colony-stimulating factor (rhG-CSF) administration on neutropenia induced by cyclophosphamide (CPA) were studied experimentally in a mouse model. Experimental mice were divided into three groups: (a) treatment with rhG-CSF after CPA administration (post-treatment group); (b) treatment with rhG-CSF both before and after CPA administration (pre- and post-treatment group); and (c) treatment with saline after CPA administration (control group). The results were as follows. Mice receiving rhG-CSF on the 2 days preceding CPA treatment, in which progenitor cell counts outside the S-phase when CPA was administered were the lowest of all the groups, showed accelerated neutrophil recovery but decreased neutrophil nadirs compared with the control group despite rhG-CSF treatment. The pre- and post-treatment group, consisting of mice who received rhG-CSF treatment on days -4 and -3 before CPA treatment, and in which progenitor cell counts when CPA was administered were increased to greater levels than in the other groups, showed remarkably accelerated neutrophil recovery and the greatest increase in the neutrophil nadirs of all the groups. These results suggested that the kinetics of progenitor cell populations when chemotherapeutic agents were administered seemed to play an important role in neutropenia after chemotherapy, and that not only peripheral neutrophil cell and total progenitor cell counts but also progenitor cell kinetics should be taken into consideration when administering rhG-CSF treatment against the effects of chemotherapy.