To investigate the sequence of cellular and protein changes after a late asthmatic reaction (LAR), bronchial lavage was carried out in 44 patients with red cedar asthma at different time intervals after bronchial challenge with plicatic acid. The results were compared to five patients with red cedar asthma who became asymptomatic after removal from exposure to red cedar for more than 2 months and 31 healthy subjects without asthma. The LAR was found to be associated with an increase in eosinophils in the lavage fluid, an increase in sloughing of bronchial epithelial cells, and an increase in degenerated cells consisting mainly of degenerated epithelial cells and alveolar macrophages. There was an increase in vascular permeability as reflected by an increase in albumin in the lavage fluid. Although there was a slight but significant increase in neutrophils 48 hours after bronchial challenge, neutrophil infiltration was not a prominent feature earlier. The potential role of loss of epithelial cells to account for an increase in nonspecific bronchial hyperresponsiveness after an LAR was discussed.