Immunoglobulin-containing cells were revealed by immunofluorescence in lung sections from mice infected with influenza virus by the aerosol route. The numbers of immunoglobulin A (IgA)- and IgM-containing cells were increasing by day 3 of the infection, whereas IgG-containing cells appeared a few days later. The responding B-cell populations appeared in two principal locations: along major airways and in consolidated lesions within lung parenchyma. IgA-containing cells were the most numerous isotype, occurring predominantly in the lamina propria of the airways. IgG-containing cells were the least frequently encountered class along airways and appeared most often within consolidated lung lesions in clustered groupings. Cells staining for mu chain appeared along the airways and in lung lesions. The population of IgM-containing cells declined approximately 30 days after infection. Cells producing alpha and gamma chains were still numerous on day 46. Assays for virus-reactive antibodies in lung secretions were positive on day 8 of the infection. The IgM titers were the first to decline, but virus-binding antibodies for all classes were still present on day 33. The implications of immune responses in viral pneumonitis were considered.