Neonatal sepsis is most often caused by group B streptococci (GBS) and is a major cause of death in the neonatal period. The response of the immune system in the newborn child has received much attention and is thought to be deficient in a number of ways. The effector response of neonatal monocyte-derived macrophages (MDM) was investigated. Interferon-γ induced the activation of indoleamine 2,3-dioxygenase in MDM and inhibited the growth of GBS. Both effects were enhanced by the addition of tumor necrosis factor-α to the culture conditions. The coincident supplementation of l-tryptophan with the bacteria abrogated the bacterial growth inhibition, thus confirming the causative role of l-tryptophan depletion. Control of the extracellular as well as intracellular l-tryptophan levels may thus be one of the effector mechanisms with which the immune system defends the host against GBS dissemination and disease.