Group B streptococcus (GBS) continues to cause considerable morbidity and death in newborn infants despite the use of antibiotics. We investigated the use of adjunctive therapies to be used with antibiotics in the treatment of neonatal sepsis, using a neonatal rat model of established GBS disease. After the development of GBS bacteremia, a human IgG preparation hyperimmune for GBS, administered with penicillin, decreased the mortality rate compared with the use of penicillin alone (14% vs 57%; p = 0.02). Similarly, recombinant human granulocyte-macrophage colony-stimulating factor, administered in a range of doses to animals with bacteremia, decreased mortality rates. The greatest effect was noted at a dose of 0.05 micrograms/kg (mortality rate 39% in combination with penicillin vs 76% for penicillin alone; p < 0.0001). Thus adjunctive therapies such as those studied here may have the potential to improve the outcome of neonatal sepsis.