Cytokine (interleukin 6 [IL-6] and tumor necrosis factor alpha [TNF-alpha]) activity in nasopharyngeal secretions of 21 infants and children (19 days to 16 months old) infected with primary respiratory syncytial virus was determined by an enzyme-linked immunosorbent assay. IL-6 and TNF-alpha were detectable in 100% (21 of 21) and 67% (14 of 21) of cases during the course of infection, respectively. Generally, TNF-alpha activity was high in the acute phase and declined thereafter, sometimes to undetectable levels. IL-6 activity was also highest in the acute phase and declined thereafter in infants younger than 5 months, while in patients older than 5 months, it-increased during the course of the disease to peak in the early convalescent phase. These observations suggest that inflammatory cytokines are produced in vivo in infants and children in response to primary respiratory syncytial virus infection and may be involved in disease pathogenesis. However, the mechanism of induction of cytokines may be different for infants and children in different age groups.