Despite improvements in infant intensive care and the use of broad spectrum antimicrobial agents, bacterial sepsis remains a major cause of morbidity and mortality in newborn infants. We and others have previously implicated the cytokines interleukin-6 and tumor necrosis factor as important mediators elaborated during the pathogenesis of infection. We hypothesized that plasma and bronchoalveolar lavage levels of chemokines are elevated during bacterial sepsis and may differentiate bacterial from viral disease.