Allergen challenge of sensitized C57BL/6 mice induced a significantly attenuated airway hyperresponsiveness to ACh in comparison with Balb/c mice. Mice were sensitized and exposed to Af and their lung function was assessed by measuring Lung Resistance (RL, panel A) and Dynamic Compliance (Cdyn, panel B) by a BUXCO online system as described. RL and Cdyn values were obtained in response to increasing concentrations of intravenous (i.v.) ACh in both strains of mice. Data are expressed as % changes from baseline. The Naïve group (open square, C57BL/6 or open circle, Balb/c) received intranasal glycerol treatment alone. The Sensitized group (closed squares, C57BL/6 or closed circle, Balb/c) received intraperitoneal (i.p.) sensitization and intranasal (i.n.) challenge with Af extract as described. Baseline RL values in the Naïve and in the Sensitized groups were 0.84 ± 0.04 and 1.25 ± 0.11 Hgmm/cmH2O/min respectively, in the C57BL/6 mice and 1.26 ± 0.03 and 1.48 ± 0.06 Hgmm/cmH2O/min, respectively in the Balb/c mice. Results are expressed as Mean ± SEM of n = 6–7. ANOVA was used to compare the dose response curves followed by Student's t test for comparisons between individual data points. *p < 0.05 Sensitized vs Naive; #p < 0.05 C57BL/6 vs Balb/c.