Previous studies have shown that the administration of immune serum globulin (ISG) before exposure to hepatitis A virus prevents infection. The precise dose needed and the duration of the protection conferred are unclear, however. In this study, ISG doses of 2 mL and 5 mL were used for preexposure prophylaxis, and their efficacies in reducing the attack rate of hepatitis A among Israel Defence Forces troops serving in field units were compared. The attack rate during the first 4 months of follow-up was low and was similar regardless of the dose administered (0.11/10, 000 and 0.15/10, 000 for 2 mL and 5 mL, respectively; P = 1.0). In the second and third 4-month intervals after immunization, attack rates were higher (but not significantly so) among soldiers given 2 mL than among those given 5 mL. Twelve months after immunization, the cumulative attack rate for hepatitis A was significantly different for the two groups (2.78/10, 000 vs. 1.30/10, 000; P <.05). Our data suggest that preexposure immunization with 2 mL of ISG is as effective as that with 5 mL in preventing hepatitis A for 4 months. The advantage of the 5-mL dose is evident 5–12 months after administration.