Development of a readily available challenge model of human respiratory syncytial virus (RSV) infection would be a useful tool for evaluation of antiviral agents and vaccine candidates. Accordingly, we evaluated a new challenge pool of RSV A2 virus, provided by the National Institutes of Allergy and Infectious Diseases. Healthy adults subjects were inoculated intranasally with various dilutions of the pool, and virus shedding, clinical symptoms, and immune response were studied. In a preliminary study of 36 randomly selected volunteers, stratified by serum neutralization titer both higher inoculum dose (4.7 log(10) TCID(50) virus) and lower antibody titer (<==10.36 log(2)), were associated with infection. In a second confirmatory study 12 of 13 (92%) subjects selected for low serum neutralizing activity shed virus after challenge with the high inoculum of virus. Mean peak virus shedding was 2.2 log(10) TCID(50)/mL nasal wash with a mean duration of 4 days. None of the subjects developed signs or symptoms of lower airway disease, although respiratory symptom scores and nasal mucus weight were temporally correlated with virus shedding. Prescreening for low levels of neutralizing antibody may allow selection of subjects with relatively higher susceptibility to experimental infections. This model provides a safe and efficient mechanism for proof-of-concept studies of anti-viral agents and RSV vaccines, as well as for investigation of immune responses to infection.