Hepatitis B immunoglobulin and vaccine were given as post-exposure prophylaxis to 73 women after an outbreak of hepatitis B due to in vitro fertilization treatment. The immunization schedule consisted of 5 ml of hepatitis B immunoglobulin (125 IU/ml) at months 0 and 1 and recombinant hepatitis B vaccine (10 micrograms of HBvaxDNA) at months 0, 1, 2 and 6. The safety and immunogenicity of hepatitis B vaccine were studied in 16 women who became pregnant after in vitro fertilization; 57 non-pregnant women receiving the same treatment served as controls. Blood samples were drawn at 0, 1, 2, 6 and 7 months. One patient had a clinical abortion 2 days after initial immunization; other side effects of vaccination were not found in vaccinees or in their offspring. All vaccinees exhibited antibodies against hepatitis B surface antigen after vaccination but relatively low peak geometric mean titers of 258 IU/l and 684 IU/l were attained in pregnant and non-pregnant women, respectively. There were no significant differences in seroconversion rates and geometric mean titers between the two groups although the immune response to hepatitis B vaccine was slower and lower in pregnant women at all times. Our results suggest that when post-exposure prophylaxis for hepatitis B infection is indicated, passive active immunization can be started safely during pregnancy. The relative weak response to the vaccine calls for monitoring of the anti-HBs 1 month after the initial series of vaccinations.