Allergic responses are characterized by the production of Ag-specific IgE Abs that are dependent upon Th2-mediated T cell help. We determined whether heat-killed Brucella abortus (BA), an inducer of Th1 responses, could influence the allergic Th2-mediated IgE response to OVA adsorbed to alum (O/A). BA plus O/A, but not O/A alone, induced high levels of mRNA for IFN-gamma and IL-12 promptly after injection. Furthermore, initial treatment with BA plus O/A rendered both BALB/c and C57Bl/6 mice incapable of mounting high IgE responses even after repeated challenges with allergen alone. Long term abrogation of anti-OVA IgE correlated with an increased frequency of IFN-gamma-secreting OVA-specific cells and a decreased frequency of IL-4-secreting OVA-specific cells. Initial treatment with anti-IL-12 prevented BA-induced early IFN-gamma production and secondary IgG2a responses, but did not abrogate IgE suppression. Additionally, secondary OVA-specific IgE responses were down-regulated by BA conjugated to OVA or by BA given with O/A. BA-induced down-regulation of secondary IgE responses was associated with increased frequency of Ag-specific IFN-gamma-secreting cells. These results suggest the possibility that even recall Th2-mediated immune responses can be attenuated if Ag is given with a carrier or adjuvant that induces potent Th1-promoting cytokines.