We investigated the capacity of heat-killed Listeria monocytogenes (HKL), a potent stimulator of the innate immune system, as a vaccine adjuvant to modify both primary and secondary Ag-specific immune responses. Mice immunized with the Ag keyhole limpet hemocyanin (KLH) mixed with HKL generated a KLH-specific primary response characterized by production of Th1 cytokines and large quantities of KLH-specific IgG2a Ab. Moreover, administration of KLH with HKL as an adjuvant reversed established immune responses dominated by the production of Th2 cytokines and high levels of KLH-specific IgE and induced a Th1-type response with high levels of IFN-γ and IgG2a and low levels of IgE and IL-4. Neutralization of IL-12 activity at the time of HKL administration blocked the enhancement of IFN-γ and reduction of IL-4 production, indicating that IL-12, induced by HKL, was responsible for the adjuvant effects on cytokine production. These results suggest that HKL as an adjuvant during immunization can successfully bias the development of Ag-specific cytokine synthesis toward Th1 cytokine production even in the setting of an ongoing Th2-dominated response. Thus, HKL may be clinically effective in vaccine therapies for diseases such as allergy and asthma, which require the conversion of Th2-dominated immune responses into Th1-dominated responses.