Groups of infants and children with infections due to respiratory syncytial virus (RSV) were tested at the onset of illness and one, two, and five months later for the presence of RSV-specific cell-mediated immunity, as determined by in vitro lymphocyte transformation (LTF) activity, and for complement-fixing (CF) antibody. An LTF response specific for RSV was observed in 65 % of patients, and the response was largely associated with E-rosette-forming thymus-derived (T) lymphocytes. A significant cell-mediated immune response was observed in 78% of patients six months of age or younger, whereas only 46% of patients older than six months exhibited such responses. The difference between the mean stimulation index of the maximal LTF response in the two age groups was statistically significant (P less than 0.05). On the other hand, a fourfold or greater rise in titers of CF antibody was observed in only 22% of patients six months of age or younger, whereas significant rises in antibody activity were observed in 50% of subjects older than six months. In view of the fact that RSV infection is most severe in the young infant, these observations raise the possibility that cell-mediated hypersensitivity reactions may be related to the pathogenesis of RSV infection of infants.