Granulocyte-macrophage colony-stimulating factor (GM-CSF) (250 microg/m2) was administered subcutaneously to 7 normal volunteers for up to 14 days to study its effects on neutrophil kinetics and function. With treatment, blood neutrophil counts rose gradually to peak at 3 1/2 times baseline by day 14. At day 5 marrow mitotic cells were increased and post-mitotic cells decreased, and the transit time through the post-mitotic marrow pool accelerated (normal = 6.4 days, GM-CSF = 3.9 days; P < 0.01). Treatment had little effect on the blood neutrophil half-life (normal = 9.6 +/- 1.3 hours; GM-CSF = 13.1 +/- 2.4 hours, P > 0.05); or the neutrophil turnover rate (normal = 78.5 +/- 11.9 x 10(7)/cells/kg/day, GM-CSF = 91.4 +/- 19.8 x 10(7)/cells/kg/day, P > 0.05). GM-CSF reduced the number of neutrophils migrating to skin chambers (normal = 104 +/- 25.0 x 10(6)/cells, GM-CSF = 48.6 +/- 16.0 x 10(6)/cells; P < 0.05). Treatment increased expression of CD11b/CD18 but not Fcgamma receptors (CD16, CD32, CD64). Treatment also stimulated the in vitro neutrophil respiratory burst in response to a variety of agonists, and this enhancement persisted for the duration of treatment. All subjects experienced local and systemic adverse effects and developed eosinophilia. This study indicates that GM-CSF at a dose of 250 microg/m2 causes neutrophilia chiefly by accelerating delivery of neutrophils from the marrow to the blood and by decreasing migration from the blood to the tissues, with only a modest effect on neutrophil production and blood half-life.