The effects of the pulmonary toxicant, 4-ipomeanol (4-IP), on Sendai viral pneumonia was examined in young adult female C57BL/6J mice. The histologic severity of the pneumonia was closely correlated with increasing doses of the compound. The more severe pneumonia observed in the 4-IP-treated animals was associated with higher pulmonary viral titers and a more diffuse distribution of viral antigen in bronchioles and alveolar parenchyma. There was no difference in the systemic humoral immune response against Sendai virus or pulmonary interferon production in animals that had received 4-IP or vehicle pretreatment. Classification of the antigen-positive cell types in the alveolar parenchyma resulted in a marked relative increase in the numbers of antigen-positive macrophages, as compared with other antigen-positive cell types.