Vaccination with a respiratory syncytial virus (RSV) fusion protein subunit vaccine (PFP-2) was done to determine if this vaccine induced evidence of cell-mediated immunity to RSV and if cell-mediated immunity prevented RSV reinfection. Healthy children 12-18 months old received 50 micrograms of PFP-2 or a saline control. Lymphocyte transformation (LTF) responses were determined before and 1 and 6 months after vaccination. PFP-2 induced positive LTF responses in 5 (83%) of 6 subjects whose prevaccination samples lacked evidence of cell-mediated immunity. Positive LTF responses in prevaccination samples were not boosted but were more persistent in vaccinees than in controls. Positive LTF responses were not associated with protection against subsequent infection. Immunization with PFP-2 induces correlates of cell-mediated immunity, but this immunity does not appear to be a critical component of protection.