Animal infectivity models have been important in the demonstration of enhanced susceptibility to viral and bacterial infection as a result of low-level toxicant exposure. The study demonstrated an enhanced and prolonged viral infection using an influenza virus infectivity model in the rat following exposure to the toxicant gas phosgene. Fischer-344 rats exposed to either air or a sublethal concentration of phosgene demonstrated peak pulmonary influenza virus titers 1 d after infection. Virus titers in rats exposed to air declined rapidly falling below detectable levels by 4 d after infection. However, a significantly enhanced and prolonged pulmonary influenza virus infection was observed on d 3 and 4 after infection in rats exposed to phosgene. Virus was cleared below detectable limits on d 5 after infection in animals exposed to phosgene. Thus, inhalation of sublethal concentrations of phosgene resulted in an increased severity of pulmonary influenza virus infection. The study provides a demonstration of the effective use of a rat viral infectivity model to detect the immunotoxicity of inhaled pollutants. The model will allow future studies to focus on the immunological mechanism(s) responsible for the enhanced and prolonged pulmonary influenza virus infection. (Copyright (c) 1991 by Hemisphere Publishing Corporation.)