Introduction: Although it is well known that neutrophils crucially contribute to host defense against invading bacteria, it has been indicated that activated neutrophils during sepsis may cause organ dysfunction and may be, on balance, deleterious. G-CSF stimulates neutrophil differentiation and function. The aim of this study was to investigate the effect of treatment with G-CSF in a murine model of polymicrobial peritonitis and sepsis induced by cecal ligation and puncture (CLP).Materials and Methods: CD1 mice received daily administration of murine recombinant G-CSF (100 μg/Kg s.c.) starting 3 days before CLP and continuing for six days after.Results: Three-day G-CSF pretreatment induced a 30-fold increase in the number of circulating neutrophils. Mortality was reduced from 96% in the vehicle-treated mice (n = 23) to 58% in the mice treated with G-CSF (n = 24, p < 0.01). G-CSF-treated mice showed a reduction of bacterial colony forming units, which were recovered from the spleens 2 (10-fold reduction, p < 0.05), 8 (40-fold reduction, p < 0.01) and 24 (80-fold reduction, p < 0.01) hours after CLP, and an increase in the peritoneal fluid neutrophils, which were collected 24 hours after CLP (5-fold increase, p < 0.01). Levels of TNF in serum and in the lungs were both decreased by G-CSF treatment to 40% of control treatment (p < 0.05 for both). Similarly, serum levels of nitrites/nitrates (indices of NO) and of the acute phase protein serum amyloid A were reduced by treatment with G-CSF to 63% (p < 0.05) and 54% (p < 0.01), respectively.Conclusion: These results show that treatment with G-CSF started before CLP has beneficial effects on ensuing infection determining improvement in survival and favouring bacterial clearance and reduction of inflammation. Thus, G-CSF-induced neutrophils appear to contribute to host defense and not to be deleterious during polymicrobial peritonitis and sepsis following CLP.