The effect of granulocyte colony-stimulating factor (G-CSF) on circulating neutrophil counts in neutropenic animals has been investigated. Neutropenia was induced by a single administration of 200 mg/kg of cyclophosphamide (CFA) via the intraperitoneal route. CFA-treated male C57BL/6 mice (Charles River Italia) received single daily dosages of G-CSF (Granulokine, Roche S.p.A.) via the subcutaneous route for seven consecutive days, starting from the day following CFA administration. Dosages of 10 and 80 mg/kg/day of G-CSF were used. Data obtained in this study showed that a treatment and dosage-related recuperation from a CPA-induced leucopenia occurred following both treatment regimes. Marked increases (2 to 5 fold) in the total number of leucocytes were measured in G-CSF-treated animals at the measurements performed after 5 and 8 days of treatment. The increased leucocyte counts were almost entirely accounted for by an elevation in the number of granulocytes. Suspension of the G-CSF treatment was followed by a return of the granulocyte count to control level seven days later. The selective neutrophilia induced by G-CSF treatment was accompanied by an increase in extramedullary haematopoiesis in the spleen as shown by a significant increase in the number of granulocyte/macrophage colony forming units.