Immune globulin is effective about 85% of the time in preventing hepatitis A in people who have been exposed, if it is given within 14 days of exposure. Several lines of evidence suggest that hepatitis A vaccine might also be effective in this setting, and vaccine has the advantage of providing long term protection. In this study, we compare how well immune globulin and hepatitis A vaccine work in preventing clinical hepatitis A in household contacts of persons with the disease. The study's hypothesis is that the the proportion of exposed household contacts who receive hepatitis A vaccine within 14 days of exposure and develop hepatitis A disease will be similar to the proportion of exposure household contacts who receive immune globulin within 14 days of exposure and develop hepatitis A disease.