Phagocyte respiratory burst capacity in response to pathogenic fungi and the in vitro effects of granulocyte colony-stimulating factor (G-CSF) were examined in 15 normal volunteers and 39 transplant recipients (4 autologous and 4 allogeneic bone marrow, 3 heart, 10 liver, 8 lung, and 10 kidney). Chemiluminescence was measured for reaction mixtures of diluted whole blood, opsonized fungi, and luminol, with and without in vitro incubation with r-metHuG-CSF (Filgrastim). Transplant patients exhibited significantly impaired respiratory burst responses to conidia and yeast compared with controls, but this was reversed with Filgrastim. Responses to hyphae were low for both groups, and G-CSF had little or no effect. There was excellent correlation between responses to fungi and responses to opsonized zymosan. In vitro respiratory burst capacity is impaired in transplant recipients. This may predict susceptibility to invasive fungal infections. G-CSF can reverse impaired phagocyte function and is of potential benefit in the prevention and/or treatment of fungal infection in transplant patients.