The effects of NO2on immune reponses of mice were investigated. Mice were exposed to various concentrations of NO2in inhalation chambers. After exposure the following parameters were measured: phagocytosis of polystyrene beads by both peritoneal and alveolar macrophages, production of antibody‐forming cells from mice immunized with sheep erythrocytes, lymphocyte blastogenesis of splenic cells, and susceptibility to influenza virus. The production of antibody‐forming cells was reduced in mice that were exposed to 5 ppm NO2. The serum antibody titers, phagocytosis, and other immune parameters measured were not affected. Exposure to NO2did not affect mortality to influenza virus. These data indicate that certain immune parameters were altered by exposure to NO2however, NO2does not appear to be a major immunosuppressive factor at the concentrations tested.