Infections with respiratory syncytial virus (RSV) may cause substantial morbidity and increased mortality in neonates and infants, especially those born before term and/or with pulmonary or cardiac disease. Humoral immunity was long thought to play only a small role in prevention, reduction of severity, or recovery from RSV infections of the lower respiratory tract. We recently performed a series of experiments in RSV-infected cotton rats, owl monkeys, and human infants to assess the role of humoral immunity in the prevention or recovery from RSV infections. This report summarizes studies utilizing intravenous IgG (IVIG) administered parenterally or topically (into the lower airway) for prophylaxis or treatment of RSV infections of the lower respiratory tract. The prophylactic administration (parenteral or topical) of IVIG to cotton rats or its therapeutic administration to RSV-infected cotton rats, owl monkeys, or humans significantly reduced RSV replication. The reductions in titers of virus correlated positively with titers of RSV-neutralizing antibody. Clinical trials are indicated to determine whether parenteral IVIG prophylaxis might prevent serious RSV infections in high-risk infants.