Abstract： Objective To analyze the correlation of the concentration of antibodies against hepatitis B surface antigen (anti-HBs) between normal full term pregnant women and their newborns.Methods Sixty-three paired serum samples from infants and their anti-HBs positive mothers,who delivered in the Affiliated Drum Tower Hospital of Nanjing Medical University School from Dec.2006 to Feb.2007,were quantitatively tested for anti-HBs by AxSYM AUSAB immunoassay.Subclasses of anti-HBs IgG were analyzed.The neutralizing activity of maternal anti-HBs in infants was assessed by surrogate assay. Results All infants born to anti-HBs positive mothers were also seropositive.Anti-HBs 1evels in infants were correlated with the maternal concentration(rs=0.976,P<0.01),and majority of the infants had higher concentrations than their mothers(195.4 mIU/ml vs 141.6 mIU/ml,P<0.01).The mean half-life of maternal anti-HBs in infants was estimated to be 41 days.Anti-HBs IgGl subclass was predominantly in infants,which was in agreement with that in their mothers.Transplacentally transferred anti-HBs in infants efficiently bound the hepatitis B surface antigen derived from hepatitis B patients. Conclusions Anti-HBs IgG carl efficiently transfer through the placenta.Infants born to anti-HBs positive mothers may be immune to hepatitis B virus in the early life of several months.