A series of complex immunological changes occur in the lungs of mice infected with Sendai virus. Early in the infection a rapid increase occurs in the number of immunoglobulin-containing cells in the vicinity of infected mucosal cells. Qualitative and quantitative changes occur in the immunoglobulins and other serum proteins in the bronchial secretions at the time of the inflammatory response. High immunoglobulin levels in the bronchial secretions persist long after the inflammatory changes have subsided. The concentrations of detectable specific antibody do not increase until after the commencement of the viral eradication process.