The serum antibody response to simultaneous administration of immune globulin (Ig) and an inactivated hepatitis A vaccine was investigated in healthy volunteers who had been tested and found free of hepatitis A virus. One hundred and forty nine subjects were randomly allocated into three groups. Group 1 received three doses of hepatitis A vaccine at 0, 1 and 6 months, group 2 received 5 ml of Ig and group 3 received a combination of Ig and vaccine. In group 3 the seropositivity rate measured by enzyme-linked immunosorbent assay was 100% at day 5, month 1 and 2, 96% at month 6 and again 100% at month 7. In the group that received vaccine alone the seroconversion rates were 0, 96, 100, 98 and 100 respectively. The geometric mean titres in subjects who received passive/active immunization were about twofold lower than in subjects who received vaccine alone, indicating interference of Ig with the immune response. Despite this, the data show that simultaneous administration of hepatitis A vaccine and Ig confers both immediate protection via Ig administration and long-term vaccine-induced protection. As the antibody levels reached are about twofold lower compared to that after administration of hepatitis A vaccine alone, a booster dose may be required sooner, than if the vaccine were administered alone.