An outbreak of hepatitis A occurred in a religious community in Indiana, USA. Sixty-nine cases were ascertained among the 4466 residents over a year, and the highest attack rate was in children. The management of the outbreak included the widespread use of prophylactic immune serum globulin (ISG). Despite this, further cases occurred. To guide further ISG administration, a survey was undertaken to ascertain what proportion remained susceptible to HAV infection. From a random sample of 600 people in the affected community 440 saliva specimens (73%) were obtained. Of these, 12·5% were found to be immune (95% confidence intervals from 9–16%). No changes were made to the ISG administration policy. There was no evidence to suggest that administration of ISG had any effect on the duration of the outbreak. There was a low rate of symptomatic infection among young children (less than 10 years); as ISG does not prevent the spread of the virus its use is not recommended in future outbreak situations.