The prevalence of asthma has steadily increased during the last decade, probably as the result of changes in the environment, including reduced microbial exposure during infancy. Accordingly, experimental studies have shown that deliberate infections with live pathogens prevent the development of allergic airway diseases in mice. Bacterial extracts are currently used in children suffering from repeated upper respiratory tract infections. In the present study, we have investigated whether bacterial extracts, commercially available as Broncho-Vaxom (BV), could prevent allergic airway disease in mice. Oral treatment with BV suppressed airway inflammation through interleukin-10 (IL-10)-dependent and MyD88 (myeloid differentiation primary response gene (88))-dependent mechanisms and induced the conversion of FoxP3 (forkhead box P3)(-) T cells into FoxP3(+) regulatory T cells. Furthermore, CD4(+) T cells purified from the trachea of BV-treated mice conferred protection against airway inflammation when adoptively transferred into sensitized mice. Therefore, treatment with BV could possibly be a safe and efficient strategy to prevent the development of allergic diseases in children