In order to investigate the possible role of immunity in lower respiratory tract disease of infants produced by respiratory syncytial (RS) virus, 18 hospitalized infants were tested for cell-mediated immune (CMI) responses in a whole blood culture assay utilizing a gamma emitting tracer, 5(125I) Iodo-2'-deoxyuridine [125IUdR] to quantitate cellular proliferative responses to virus antigen. Class-specific antiviral antibody titres were determined in an indirect membrane immunofluorescence test. One infant showed a CMI response in the acute phase of illness whereas 72% responded one month later. Of the 18 infants, 14 were tested for antibody responses and 71% showed significant rises of antiviral IgG. IgM was detectable in only one acute phase specimen. A tendency for higher CMI responses following severe infection with RS virus was noted but little difference in antibody responses was respect to severity was seen. These findings are discussed in relationship to the pathogenesis of RS virus.