Systemic cell-mediated responses to respiratory syncytial (RS) virus were detected, using a whole blood transformation assay, in 10 of 28 infants and children with RS virus infections during the period 1-14 days postadmission. Cell-mediated responses were unrelated to the age of the patient or the severity of illness. No correlation was found between cellular responses and fourfold or greater rises in antibody titre to RS virus, as determined by a membrane immunofluorescence technique. Patients under 6 months of age had significantly lower levels of IgA and IgG antibody to RS virus compared to older patients, although cell-mediated responses were similar in both groups. The presence of cell-mediated reactivity to RS virus was also demonstrated in 5 of 95 samples of cord blood examined, and cellular responses failed to correlate with the levels of IgG antibody to RS virus.