In order to obtain the beneficial effects from granulocyte-macrophage colony-stimulating factor (GM-CSF) on granulo-monocyte recovery with the minimum dose and toxicity, we compared the effect of two different GM-CSF schedules (5 micrograms/kg/day subcutaneously, days 5 to > 18 versus days 12 to > 18 on the cytopenias which follow cytostatic treatment with carboplatin (400 mg/m2 intravenous (i.v.) day 1) and etoposide (100 mg/m2 i.v. days 1 to > 3). 13 patients entered the study for a total of 36 evaluable cycles. The cytostatic treatment produced a neutropenia that persisted for up to day 22 (absolute neutrophil count (ANC) < 1000/microliters in 25% and ANC < 2000 in 50% of control cycles). Early GM-CSF administration markedly increased the leucocyte nadir and produced two waves of leucocytosis: an early one, linked to marrow reserve release and presumably of no value to the patients; and a delayed one, due to marrow precursor and progenitor cell proliferation, in which the granulomonocytosis was associated with a marked eosinophilia. The delayed GM-CSF administration markedly increased the leucocyte nadir and accelerated granulo-monocyte recovery (with an only modest eosinophilia), so that chemotherapy could be repeated every 21 days in all the patients.