An increase in airway reactivity after the late asthmatic response (LAR) has been noted in humans. Although these alterations in reactivity have been proposed to be associated with inflammation, no clinical study of the LAR has shown both increased airway reactivity and evidence of pulmonary inflammation. Employing an animal model of the LAR in rabbits developed in our laboratory (Am Rev Respir Dis 1982; 126:493–498), we examined changes in airway reactivity and pulmonary inflammation in rabbits having a late asthmatic response. Two groups of rabbits were studied: a control group (n=10) received nonimmune serum, and a sensitized group (n=10) received serum containing homocytotropic antibody (IgE) to ragweed extract (RWE) from rabbits previously immunized from birth with ragweed. Airway reactivity to histamine and the evaluation of different cell types in bronchoalveolar lavage fluid were determined 3 days before and 3 days after bronchial challenge with RWE in all rabbits. No control rabbit developed a LAR, and no significant changes occurred in this group's airway reactivity or cells in lavage fluid after bronchial challenge with RWE. In contrast, all sensitized rabbits developed a LAR, and airway reactivity for this group was markedly increased 3 days later. In addition, the total number of cells including both polymorphonuclear and mononuclear cells in lavage fluid increased significantly 3 days after the LAR. Ten days after the LAR in the sensitized group (n=8), as airway reactivity returned towards baseline, so did the cells in lavage. We conclude that increased airway reactivity to histamine is temporally associated with pulmonary inflammation as defined by cells in lavage after the LAR in this model.