Dexamethasone is frequently used antenatally or postnatally to prevent or treat neonatal disease. However, its use has been associated with side effects including an increased risk of infection. Previous reports about the effect of corticosteroids on neutrophil phagocytic function are conflicting. GBS is a significant cause of early-onset infection in the neonate and neutrophil mediated bacterial killing is an important defense against this infection. We hypothesized that dexamethasone concentration would affect neutrophil mediated bacterial killing and studied this in-vitro. Polymorphonuclear neutrophils (PMNs) were isolated from healthy adult volunteers (n=15) by dextran and Ficoll-Hypaque density centrifugation and resuspended to a final concentration of 2.5 × 107 cfu/ml. PMNs were then either: a) incubated for two hours with various concentrations (0, 0.01, 1.0, 100, 1000, 2000 mcg/ml) of dexamethasone in the presence of GBS (1 × 106 cfu/ml, serotype III, strain SS 878), newborn rabbit complement, and increasing dilutions of human antibody; or b) incubated with the same concentrations of dexamethasone for one hour, washed and resuspended to a final concentration of 2.5 × 107 cfu/ml then incubated for two hours with GBS (1 × 106 cfu/ml, serotype III, strain SS 878), newborn rabbit complement, and increasing dilutions of human antibody. In both instances colony counts were performed prior to and after the final two hour incubation and bacterial killing calculated. No bacterial killing was noted in negative controls which included only bacteria, PMNs, antibody, complement, PMNs and complement, and PMNs and antibody. Results are expressed as the antibody level at which the largest reciprocal dilution promoted ≥ 90% bacterial killing. PMNs exposed to dexamethasone levels ranging from 0.01 to 100 mcg/ml during the bacterial killing assay demonstrated killing at higher antibody dilutions (20-1 versus 12-1). PMNs exposed to similar levels of dexamethasone prior to the bacterial killing assay also demonstrated killing at higher antibody dilutions (18-1 versus 11-1). While PMNs exposed to higher concentrations of dexamethasone (1000, 2000 mcg/ml) demonstrated killing at lower antibody dilutions (2.5-1). Dexamethasone concentration appears to affect the neutrophil mediated bacterial killing of GBS. Dexamethasone, at clinically observed concentrations, appears to enhance bacterial killing, while at higher concentrations appears to suppress killing. We speculate that the role of dexamethasone in neonatal infection may be more complex than previously described.