Detection of viral antigen M1 protein in the lung of H1N1- and H3N2-infected pikas, as well as in the lung and brain from H5N1-infected pikas. Immunohistochemical analyses of viral antigen distribution in the tissues of pika using a monoclonal antibody against M1 protein in (A) Uninfected lung, (B) H1N1-infected lung, (C) H5N1-infected lung, (D) H3N2-infected lung, (E) Uninfected brain, and (F) H5N1-infected brain. Arrows denote the M1-positive Purkinje cells. M1 is expressed on the membrane and in the cytoplasm of epithelial cells of the small bronchi, terminal bronchioles, and alveolar cells. Bar, 100 μm.