The bacterial extract OM-85-BV prepared from 21 pathogenic bacterial strains is administered orally to adults and children for the treatment and prevention of recurrent infections of the respiratory tract. We analyzed in vitro and in vivo the immunomodulatory effects of the extract. The lysate acted as a non specific macrophage activator, inducing NO production as well as the translocation of transcription factor NF-kappaB into the nucleus in murine bone marrow-derived macrophages. Besides stimulating unspecifically the immune system, a bacteria-specific humoral immune response was revealed. After oral application, a trend to increase bacteria-specific IgG and IgA in serum was observed. Also a marked increase of bacteria specific IgA in saliva as well as in supernatants of Peyer's patches and mesenteric lymph nodes-derived cell cultures was found. The immunomodulatory properties of the extract were also investigated with respect to shifting the Th1/Th2 bias in an in vivo allergy model. BALB/c mice were orally immunized with OM-85-BV and subsequently sensitized intraperitoneally with the allergen ovalbumin. The group pretreated with OM-85-BV showed a decrease of both total and ovalbumin specific IgE. Accordingly, in spleen cell supernatants the Th1-associated cytokine IFN-gamma was increased, and the Th2-associated cytokine IL-4 was downregulated. Our findings suggest that the immunoprotective effects of OM-85-BV observed in human beings may be correlated to its Th1 augmenting properties.