The defective ability of human newborns to mobilize phagocytes to the site of infection led us to examine the ability of cord blood mononuclear cells to secrete interleukin-8, a major neutrophil chemotactic factor, in response to stimulation with Listeria monocytogenes. Adult or cord blood mononuclear cells were incubated with L. monocytogenes for varying lengths of time, and IL-8 was measured in the culture supernatants by the enzyme-linked immunosorbent assay (ELISA). Spontaneous IL-8 secretion by unstimulated cells was undetectable or at the minimal detection limit of the assay. By 24 h of cell incubation with L. monocytogenes, newborn cells produced as much IL-8 as adult cells did (300 +/- 113 versus 269 +/- 189 ng/ml, respectively). Over the next 2-4 days, IL-8 output by adult cells was slightly higher than that by newborn cells, but the difference was not statistically significant. The in vitro results suggested that newborns are as able as adults to produce IL-8, although they are defective in mobilizing neutrophils, the IL-8 target cells, to the site of infection.