The efficacy of modified-live (MLV) bovine respiratory syncytial virus (BRSV) vaccines and the correlates of vaccine-induced immunity were investigated in calves using a virulent experimental infection. Clinical disease and pulmonary pathology were significantly reduced, relative to unvaccinated controls, in calves vaccinated according to label directions with commercial multivalent MLV BRSV vaccines. In vitro assays of cellular immunity were more consistent correlates of vaccine associated protection than presence of post vaccination serum antibody. Most vaccinated calves shed virus, but peak virus titre was suppressed compared to unvaccinated controls, with clearance coincident with the simultaneous appearance of mucosal antibody, cytotoxic cells in the lung and anamnestic or primary serum antibody responses. Virus clearance in unvaccinated calves was coincident with the appearance of BRSV specific cytotoxic cells, before mucosal antibody was detected.