The report consists of four studies. The materials, methods, and experimental protocol for each study were presented. The parameters of interest were hemagglutination-inhibition (HI) and serum neutralization (SN) antibody formation, serum immunoglobulin levels, lung histopathology, mortality rates, lung lesion scores, and extent of lung edema in mice. The four studies were: immune response in mice during long term exposure to nitrogen dioxide, immune response in mice during a four-week exposure to NO2, effect of chronic exposure to NO2 on resistance to Klebsiella pneumoniae, and effects of manganese on resistance to respiratory infection. In the first study prior to vaccination the mice were exposed continuously to 2ppm NO2, 0.5ppm NO2 with one-hr daily peaks of 2ppm NO2 5 days in a week, or filtered air for three months. They were thereafter held in either an NO2 environment or filtered air.