Abstract: Both the positive seroconversion rate and titer of anti HBs(Miu/ml) in the newborns who have been vaccinated with plasma derived hepatitis B vaccine in 1988～1992 have decreased markedly.About 75 percent of the newborns' antibody titer of anti-HBs was less than the protective titer level (<10 Miu/ml).But there was no significant difference in the positive seroconversion rate of HBsAg between pre-vaccination and one year after vaccination. It still has a good protective effect.After one month of vaccination with one dose(5μg)recombined yeast hepatitis B vaccine boost,the positive seroconversion rate and titer of anti-HBs have risen by 28.60% and 20.9% respectively.There was good recall reaction with boost vaccination in the children whose anti-HBs titer were less than 10 mIU/ml.