The use of granulocyte-colony stimulating factor (G-CSF) for the recovery from neutropenia has been established; however, acute lung injury due to G-CSF induced polymorphonuclear leukocytes (PMN) activation is a serious complication. This study was designed to compare the activation of PMN with single bolus administration and continuous administration of G-CSF. Healthy volunteers (age, 33.8 ± 1.4 years, n = 6) received a single bolus injection of 50 micrograms/m² of G-CSF (SI; n=6) or continuous subcutaneous injection of 50 micrograms/m² of G-CSF for 24 hrs (CI; n=6), and were followed for 48 hrs. Circulating leukocyte counts, markers of activation on PMN, and circulating levels of G-CSF, IL-6 and PMN elastase were measured. SI rapidly increased serum G-CSF levels that peaked at 4 hrs, whereas CI gradually increased G-CSF levels, which remained at a steady level from 8 to 24hrs. SI caused a rapid decrease in PMN counts at 0.5 hr followed by sustained increase to peak at 12 hrs. CI gradually increased PMN counts, which peaked at 24 hrs, but the peak values were not significantly different between the groups. SI induced activation of PMN, which was characterized by increased expression of CD11b, decreased expression of L-selectin, and increased F-actin content, lead to increases in serum IL-6 and PMN elastase level. Such changes were all attenuated with CI (p < 0.05). We conclude that continuous subcutaneous injection of G-CSF resulted in a similar marrow response as a single injection but yielded reduced PMN activation.