Previous work with a neonatal ferret model for human SIDS had indicated that inflammation caused by a combination of influenza virus and bacterial endotoxin may be a cause of human SIDS. To determine whether cytokines may be involved in this inflammatory response, levels of interleukin (IL)-1 beta, IL-6 and tumour necrosis factor (TNF)-alpha were examined, using ELISA assays, in culture supernatants of human peripheral blood leucocytes infected with influenza virus and subsequently incubated with endotoxin. Levels of TNF-alpha were increased compared to cells incubated with virus or endotoxin alone. Levels of IL-1 beta were also increased whereas levels of IL-6 were generally not enhanced. Cytokines appeared within 1-2 h of stimulation with virus or endotoxin and increased subsequently to reach maximum titres between 16 and 20 h post treatment. While levels of cytokine were much lower when determined using bioassays rather than ELISA assays, the pattern of increased yields from cells incubated with virus and endotoxin compared with either alone was still evident. The possible importance of these observations for SIDS victims is discussed.