Neonatal granulocytes are recognized to have functional defects which are thought to be important in the increased susceptibility to infection in the neonate. Recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF), a member of a family of glycoproteins essential for the in vitro survival, proliferation, and differentiation of hematopoietic progenitor cells, has been shown to enhance the functional capabilities of adult granulocytes. This study tested the effects of rhGM-CSF on the locomotion, superoxide generation, phagocytosis, bactericidal activity, and membrane depolarization responses of cord blood granulocytes. Concentrations of rhGM-CSF between 100 and 1 pM significantly enhanced the leading front of cord blood granulocyte locomotion. The mean distance migrated by the cell population and the number of cells responding to the chemoattractant were also significantly enhanced in cord blood granulocytes treated with 1 pM rhGM-CSF. Superoxide anion production was significantly enhanced in cord blood granulocytes stimulated with fMLP after a 30- or 60-min exposure to 100 pM rhGM-CSF. However, this enhancing effect was not observed in cells incubated with rhGM-CSF for 2 h before formyl-methionine-leucine-phenylalanine stimulation. Phagocytosis, bactericidal activity, and membrane depolarization responses of cord blood granulocytes were not affected by exposure to rhGM-CSF. These findings demonstrate that selected cord blood granulocyte functions are enhanced by in vitro exposure to rhGM-CSF. Whether these in vitro observations have in vivo significance await further study.