Granulocyte colony-stimulating factor (G-CSF) not only increases neutrophil (polymorphonuclear leukocyte, PMNL) production but also modulates PMNL biologic function. To assess the ability of G-CSF administered in vivo to enhance PMNL activity against opportunistic fungal pathogens, the antifungal activity of PMNL obtained from normal human volunteers before and after G-CSF administration was compared. In vivo, G-CSF significantly enhanced PMNL-mediated killing of Aspergillus fumigatus and Rhizopus arrhizus by 4-fold and 15-fold, respectively (P < .05). In contrast, PMNL-mediated killing of Candida albicans was unaffected by G-CSF. The ability of aqueous fungal extracts to induce the PMNL respiratory burst was evaluated by luminol-enhanced chemiluminescence. G-CSF in vivo primed PMNL for sustained chemiluminescence in response to extracts of Candida, Aspergillus, and Rhizopus organisms. These data demonstrate that G-CSF in vivo augments antifungal activities of PMNL, thereby implicating a possible therapeutic role for G-CSF as a biologic response-modifying agent during opportunistic fungal infection.