A prospective double-blind study was performed in 107,803 US soldiers assigned to Korea to determine if prophylactic injections of US-derived human serum gamma globulin prevented icteric endemic viral hepatitis abroad. Each soldier received an intramuscular injection containing either 2, 5, or 10 ml of gamma globulin or a placebo solution upon arrival in Korea and 65% received a second injection of the same material five to seven months later. Gamma globulin prophylaxis significantly decreased the incidence of hepatitis and provided passive protection for about six months. The 5ml dosage produced maximal protection. Similar protection occurred in patients with both Australia antigen (HAA)-positive and HAA-negative endemic hepatitis. The severity of illness was slightly diminished in the gamma globulin-protected group. Prophylactic injections of gamma globulin did not significantly alter the incidence of other commonplace infectious diseases.