Probiotics are normal inhabitants of the gastrointestinal tract of man and are widely considered to exert a number of beneficial effects in many diseases. But the mechanism by which they modulate the immune system is poorly understood. The present study was planned to explore the anti-allergic effect of Lactobacillus gasseri on a mouse model of allergic asthma. Dermatophoides pteronyssinus (Der p) sensitised and challenged BALB/c mice were orally administered via oral administration with three different doses of L. gasseri (low, 1 × 10(6) colony-forming units (CFU); medium, 2 × 10(6) CFU; high, 4 × 10(6) CFU), in 700 μl of PBS daily, starting from 2 weeks before Der p sensitisation for 4 weeks. After the allergen challenge, airway responsiveness to methacholine, influx of inflammatory cells to the lung, and cytokine levels in bronchoalveolar lavage (BAL) fluids and splenocytes culture were assessed. Our results showed that oral administration of a high dose of L. gasseri (4 × 10(6) CFU) decreased airway responsiveness to methacholine, attenuated the influx of inflammatory cells to the airways and reduced the levels of TNF-α, thymus and activation-regulated chemokine (TARC) and IL-17A in BAL fluids of Der p-sensitised and -challenged mice. Moreover, L. gasseri decreased IL-17A production in transforming growth factor-α and IL-6 stimulated splenocytes and cell numbers of IL-17 producing alveolar macrophages in L. gasseri-treated mice as compared to non-treated, Der p-sensitised and -challenged mice. In conclusion, oral administration with L. gasseri can attenuate major characteristics of allergen-induced airway inflammation and IL-17 pro-inflammatory immune response in a mouse model of allergic asthma, which may have clinical implication in the preventive or therapeutic potential in allergic asthma.