The most common ubiquitous air pollutants, as well as some point source (e.g. metals) air pollutants, decrease the function of pulmonary host defense mechanisms against infection. Most of this knowledge is based on animal studies and involves cellular antibacterial defenses such as alveolar macrophages and mucociliary clearance. Information on viral infectivity is more sparse. Since there is no routine treatment for viral infections which have a relatively high rate of occurrence, this gap in knowledge is of concern. Given the major gaps in knowledge, reseasonably accurate assessment of the immunotoxicity of air pollutants is not possible. When the limited data base is reviewed relative to ambient levels of the common pollutants, it appears that acute exposures to O3 and H2SO4 and chronic exposures to NO2 are the major exposures of concern for immunotoxic effects. It is critical to point out, however, that until information is available for chronic exposures to low levels of metals and for exposures to common organic vapors, the immunotoxicity of air pollutants cannot be assessed adequately.