Increased bronchial responsiveness has been reported at various time points following allergen challenge (AC), and may be related to the magnitude of the late response (LAR). We have studied 20 mild asthmatics, who were known to develop a late asthmatic response to inhalation of house dust mite extract (fall of > 15% from post-diluent baseline FEV1 from 2 to 7h after AC). The provocation concentration of methacholine causing a 20% fall in FEV1 (PC20 FEV1) was measured before and 24 h after challenge with house dust mite extract (HDM). The mean (SEM) change in log(PC20) was 0.08 (0.09) mg ml-1, and was not significant (P = 0.38; paired t-test). The change in PC20 for each subject was not significantly correlated with the size of LAR (r = -0.33; P > 0.05), but was significantly correlated with the absolute change from baseline FEV1 at 24 h (r = 0.67; P < 0.01). Our subjects had a high baseline responsiveness, when compared with previous studies. We suggest they may have been approaching a maximally responsive state prior to study, and allergen challenge may have had little effect in further increasing responsiveness. Exposure to allergen in late responders is not necessarily followed by an increase in non-specific bronchial responsiveness.