Inactivated hepatitis A virus (HAV) vaccine is given in a three-dose schedule, When rapid protection is needed, injection of immune globulin (IG) concomitantly with the first dose could provide passive protection until adequate active antibody response has developed. A possible effect ofIG on the immune response to the vaccine was studied in healthy volunteers; 28 received vaccine alone, and 34 received the first dose simultaneously with 5 mL of IG. A control group received hepatitis B vaccine, and a fourth group received IG alone. Four weeks after the first vaccine dose, all subjects had detectable ELISA anti-HAV antibodies. Several weeks after each vaccine dose, the geometric mean titer of antibodies was significantly lower in those who received vaccine with IG but higher than in those who received IG alone. Results for neutralizing antibodies yielded a similar trend. IfIG is given with HAV vaccine, a further booster vaccine dose may be required to ensure long-lasting immunity.