The fetus and newborn are particularly susceptible to Listeria monocytogenes infection. We used a newborn rat animal model to investigate neonatal host defense against Listeria. In this animal model, newborn (3-d-old) rats are more susceptible to L. monocytogenes than older animals. Juvenile (23-d-old) L. monocytogenes-infected rats pretreated with lipopolysaccharide (LPS) had a lower bacterial load in blood than control animals, whereas LPS pretreated newborn rats had a higher bacterial load. Because LPS is a potent inducer of tumor necrosis factor (TNF) and TNF enhances host defense against this organism in adult animals, we assessed TNF content in splenic homogenates for animals of different ages. The age at which TNF was detectable in L. monocytogenes-Infected rats corresponded to the age at which LPS became active in preventing severe bacteremia. TNF was less than 1 unit/mL in splenic homogenates taken from rats less than 8 d of age, whereas 16-d-old rats infected with L. monocytogenes 1 d earlier had greater than 80 units/mL (p less than 0.0001 for 3-d-old versus 16-d-old rats). We also assessed the responsiveness of rats to exogenous TNF-alpha. Juvenile rats pretreated with TNF-alpha before L. monocytogenes infection had decreased bacterial load in spleen (p less than 0.02 versus controls) and better survival at 7 d (p less than 0.05 versus controls), whereas newborn rats did not improve with TNF-alpha pretreatment (p greater than 0.05 treated versus controls for splenic bacterial load and 7-d survival).(ABSTRACT TRUNCATED AT 250 WORDS)