A DNA fraction purified from Mycobacterium bovis bacillus Calmette-Guérin (BCG) and designated MY-1 induced interferon (IFN)-gamma production by human peripheral blood mononuclear cells (PBMC). IFN-gamma is well known as a downregulator of IgE production. In this study we investigated whether MY-1 regulates IgE production by human PBMC in vitro. MY-1 inhibited IgE production in PBMC taken from normal donors and stimulated with interleukin (IL)-4 plus monoclonal anti-CD40 antibody, without affecting production of IgA. MY-1 enhanced production of IFN-gamma and IL-12 by PBMC. Inhibition by MY-1 of IgE production was mediated by both IFN-gamma and IL-12, since the MY-1-induced suppression was blocked by the addition of monoclonal anti-IFN-gamma antibody, monoclonal anti-IL-12 antibody or a monoclonal antibody (mAb) directed at the IL-12 receptor. MY-1 inhibited the induction of epsilon germ-line transcript by IL-4. Additionally, MY-1 inhibited spontaneous in vitro production of IgE by PBMC from atopic donors in the absence of IL-4 plus anti-CD40 mAb. These results suggest that exposure to MY-1 may be a novel strategy for the treatment of IgE-related allergic disease.