Administration of human serum immune globulin (Ig) is an effective means of protecting individuals against hepatitis A virus (HAV) infection and disease. Several large field studies have demonstrated that if given before exposure, Ig will prevent infection with HAV. Furthermore, if Ig is given during the incubation period of hepatitis A, the severity of infection may be reduced and potentially clinical infections may be converted into subclinical ones. Although uncommon, infection which occurs in the presence of circulating antibody may occasionally lead to passive-active immunity. Unfortunately, the duration of Ig protection is dose dependent, and high dose administration provides less than six months protection. Ig preparations contain HAV antibodies at levels detectable by commercial immunoassays; however, recipients of Ig do not have detectable levels of HAV antibodies when tested by the same method. Using more sensitive immunoassays and neutralization assays, low titres of HAV antibody can be detected in Ig recipients. Since Ig provides approximately 90% efficacy in preventing hepatitis A, it would appear that very low levels of HAV antibody are needed to prevent infection. Consequently, measurement of HAV antibodies elicited by HAV vaccines should provide a reasonable method to evaluate their immunogenicity and predict their efficacy.