To determine the safety and immunogenicity of hepatitis B vaccine in pregnancy, 72 pregnant Nigerians who were negative for markers of hepatitis B virus (HBV) were given two intramuscular doses of vaccine (Heptavax, Merck) in the third trimester. One month after the second dose (at T2), 84% were anti HBs positive. No significant effect was observed in the mothers or their newborns. Passive transfer of anti HBs occurred in 59% of the newborns. The antibodies disappeared rapidly in these infants and by 3 months only 23% had detectable antibodies. No HBsAg carrier status developed in this group. In contrast, the infants born to HbsAg positive mothers had a cummulative rate of HBV events of 20%. It is concluded that HBV vaccine is safe and immunogenic in pregnant females. The passive immunity conferred on the infants is of short duration. Therefore, infants in endemic areas should be vaccinated early, preferably within 3 months of birth. Vaccination of pregnant mothers may provide adequate protection before the child is vaccinated.