T lymphocytes may play a regulatory role in the development of allergic airway hyperresponsiveness (AHR). We have studied the relationship between airway responsiveness and a number of immunological changes in Brown-Norway rats sensitized intraperitoneally and repeatedly exposed to ovalbumin (OVA) aerosol. Acetylcholine provocation concentration (PC)150 (the concentration of acetylcholine causing a 150% increase of base-line lung resistance) was measured and peripheral blood and bronchoalveolar lavage (BAL) cells were collected 18-24hr after the final exposure. Total and OVA-specific IgE in serum was measured by enzyme-linked immunosorbent assay (ELISA). Mononuclear cells were analysed by flow cytometry after labelling with monoclonal antibodies against CD2 (pan T-cell marker), CD4, CD8 (T-cell subsets) or CD25 (interleukin-2 receptor). There were significant differences in PC150 (P < 0.05) and in OVA-specific IgE levels in serum (P < 0.002); CD4+ T cells expressed a significantly increased level of CD25 immunoreactivity in BAL, but not in peripheral blood, of rats sensitized and exposed to OVA, compared with saline-exposed controls (P < 0.02). There was a significant correlation between CD25 expression and BAL eosinophil numbers (r = 0.74, P < 0.001), PC150 (r = 0.63, P < 0.003) and OVA-specific IgE (r = 0.77, P < 0.001). These data suggest that activated T cells may be involved in the regulation of allergen-induced AHR in a relevant animal model of allergic asthma.