IL-6 is a cytokine with a wide variety of influences on the cells involved in immune and inflammatory responses. Defective production of IL-6 may be partly responsible for the impaired immune defense and inflammatory response often observed in the neonatal period. In our study, we used whole blood culture to examine the capacity of neonatal leukocytes to produce IL-6 in response to various stimuli. IL-6 activity was evaluated by growth promoting assay using an IL-6-dependent murine hybridoma clone. IL-6 activity was undetectable in fresh or unstimulated blood obtained from both newborns and adults. In contrast, incubation of whole blood with lipopolysaccharide or concanavalin A resulted in marked IL-6 activity. After stimulation, IL-6 activity was induced as early as 2 h after culture and increased with time, reaching a plateau at around 12 h. Comparative examinations suggested that the IL-6 activity induced in neonatal blood on stimulation was similar to that seen in stimulated adult blood. Neutralization experiments with anti-IL-6 anti-serum confirmed the presence of IL-6 proteins in the stimulated blood, and induction of cellular IL-6 mRNA was demonstrated in the stimulated blood as well. In addition, immunocytochemical observations suggested that the major IL-6 producing cells in the stimulated blood may be monocytes. The results suggest that the production of IL-6 in response to specified stimuli is normal at birth.