As a result of inadequate placental transport of maternal IgG, preterm neonates of less than 32 weeks' gestation, especially those with birth weights less than 1,500 g, are profoundly hypogammaglobulinemic at birth, a condition that worsens during the first several weeks of life. This hypogammaglobulinemia is believed to contribute to their high frequency of late-onset sepsis, with its accompanying morbidity and mortality. Animal studies suggest that human immunoglobulin prepared for intravenous use (IVIG) improves host defense against pathogens that cause neonatal infections, but studies of IVIG in human neonates have been inconclusive because of the small numbers of infants included, lack of suitable controls, use of clinical rather than strict microbiologic definition of sepsis, and performance only in a single hospital outside the United States. A double-blind, randomized, placebo-controlled multicenter trial in the United States is in progress to determine the efficacy of IVIG in the prevention of late-onset infections in infants with birth weights between 500 and 1,750 g. Infants are infused with 500 mg of IVIG/kg or albumin-saline placebo at 3-7 days of age, 7 days later, and every 14 days for five doses. Efficacy parameters include mortality, number of proved infectious episodes (bacterial, fungal, or viral), and infection-related morbidity. Definitive guidelines for the possible use of prophylactic IVIG in low-birth-weight neonates should result from this evaluation of 500 to 700 infants in the United States.