A model for studying effectors of immunity to respiratory syncytial virus (RSV) was developed. Paris of inbred cotton rats (Sigmodon hispidus) were joined surgically using the technique of parabiosis. One week later, one animal of each pair was primed intranasally with a small volume of RSV suspension. Fourteen days after priming, both animals of each pair were bled for determination of serum neutralizing antibody titers, and challenged intranasally with a standard dose of RSV suspension. Single, unprimed cotton rats were challenged concomitantly and served as controls. Four days after challenge, all animals were sacrificed for virus titration of nasal tissues and lungs. Parabiosed cotton rats were surgically separated at varying intervals between priming and challenge (days 7, 9, 12, or 14 after priming) or were kept joined until sacrificed (day 18). Significant transfer of nasal and pulmonary immunity from primed to unprimed parabionts began 9 days after priming, gradually increasing through 18 days. Resistance to RSV challenge in spite of low levels of serum neutralizing antibody suggests that non-antibody immunologic mediators were responsible for the transferred immunity. Evidence is presented for three broad categories of RSV immunologic effectors: systemic, local with a transient systemic phase, and local without a systemic phase. These categories are now amenable to further study using the described model.