Abstract: Objective To explore the relationship between placental apoptosis,peripheral blood mononuclear cell(PBMC)transfer from mother to baby and hepatits B virus surface antigen(HBsAg)of PBMC in the neonates born to HBsAg positive pregnant women.Methods Mother-baby pairs were screened with allele-specific PCR(As-PCR).Teminal deoxy nucleotidyl transferase(TdT-)mediated dUTP nick end labeling(TUNEL) method was used to detect placenta apoptosis index of HBsAg-positive full term pregnant women.Immunofluorescence double labeling method was used to detect HBsAg and glutathione S transferase(GST) in neonatal PBMC smear samples.Statistical analysis was performed by using SPSS 16.0.Results Of 86 neonatal PBMC smear samples,20(23.30%)were HBsAg positive,31(36.00%)were GST positive,and 13(15.12%)were both HBsAg positive and GST positive(15.12%),respectively.The placental cell apoptosis index was significantly different between the pregnant women with and without PBMC transfer(t'=2.38,P=0.02),and the rate of apoptosis in placetal trophoblast cells and PBMC transfer was related( t=2.75,P=0.01).Placental apoptosis index between PBMC HBsAg positive and negative neonates was not significantly different(t=0.34,P=0.74).There was a correlation between PBMC transfer from mother to baby and HBsAg positive of PBMC in neonates(χ²=9.48,P=0.00),and the risk of HBsAg positive of PBMC in the neonates with PBMC transfer was 4.95(95%confidence interval:1.70-14.39)times higher than in the neonates without PBMC transfter.Conclusion Placental apoptosis may increase placental permeability during late pregnancy,which is beneficial to the mother-baby transfer of PBMC.PBMC infected with HBV may pass into fetal blood circulation through placenta and induce HBV infection in neonates.