ObjectiveTo determine if streptococcicidal levels of penicillin G benzathine can be detected in maternal serum 4 weeks after treatment with 4.8 million units.MethodsThirty-seven pregnant women with positive group B streptococcus vaginal or urine cultures were each given 4.8 million units of penicillin G benzathine. Maternal blood samples were collected after injection and at delivery. Serum penicillin levels were measured by high-pressure liquid chromatography. Follow-up cultures were done when possible.ResultsNone of the patients had serum penicillin levels below 0.20 μg/mL 30 days after treatment. Cord blood levels were approximately 50% lower than maternal levels. In all but three subjects, cord blood levels exceeded 0.06 μg/mL, the minimal inhibitory concentration for group B streptococcus. The three exceptions were patients who delivered more than 100 days after treatment. Group B streptococcus cultures were negative at the time of delivery in 72% of cases. None of the patients with positive cultures were moderately or heavily colonized.ConclusionIn pregnant women, penicillin G benzathine levels are high enough to inhibit the growth of group B streptococcus for more than 4 weeks after injection with 4.8 million units. Further studies are needed to evaluate whether this regimen can prevent neonatal colonization and invasive group B streptococcus disease.