The ability of granulocytes and mononuclear phagocytes to phagocytose and kill S.aureus and to generate superoxide anion (O-2) upon stimulation with opsonized bacteria was studied in five patients (ages 9 to 26 years) with type I Gaucher disease (GD). Before treatment was started, bacteria were ingested equally by phagocytic cells from patients and age-matched controls. The extent of killing of S.aureus and the release of O-2 by granulocytes over 60 min incubation were also equivalent for patients and controls. However, we found that killing of viable bacteria and release of O-2 by the patients' monocytes were significantly lower than in cells from controls (p < 0.05 for both). The magnitude of differences in killing and O-2 release between patients' cells and those from controls was even more profound with macrophages (p < 0.001 for both). Enzyme augmentation with macrophage-targeted glucocerebrosidase for 6 mo at doses from 7.5 to 10 U/kg/week resulted in significant increases of functional activities and O-2 generation of monocytes and macrophages (45±9% and 6±4% killing of bacteria by monocytes and macrophages, respectively, before treatment compared to 66±12% and 13±4% killing by the two cell types after treatment, respectively, n = 5) along with hepatosplenic and hematologic improvements. These data suggest that mononuclear phagocytes from Gaucher patients are defective in their ability to kill bacteria and to generate O-2 and that enzyme substitution improves functions of monocytes and macrophages in patients with GD which should make them more resistant to severe bacterial infection.