Synthetic nonmethylated oligodeoxynucleotide containing CpG dinucleotide (CpG-ODN) has been shown to induce Th1-biasing response, which is extremely applicable in human allergic diseases. In this study, the enhancing effect of CpG-ODN on the production of Th1 associated cytokines induced by mite allergen was investigated in PBMC of the mite allergic asthmatics. PBMC from the allergic asthmatics and normal controls were isolated and cultured in vitro stimulated by CpG-ODN and Dermatophogoides farinae allergens (Df). IL-12, IFN-γ and IL-5 in the cell supernatants were detected by ELISA, whereas the cytotoxicity of CpG-ODN was detected by a tetrazolium colorimetric assay (MTT assay). Df specific IgE in sera of patients were assayed by fluorescent enzyme immunoassay. The results showed that PBMC from either the patients or normal controls stimulated with Df plus CpG-ODN led to a significantly increased levels of IFN-γ and IL-12 compared with Df plus non-CpG-ODN and Df alone, whereas IL-5 level was decreased significantly. Moreover, the levels of IFN- g and IL-12 produced by the normal controls were significantly higher than that of the patients, but IL-5 was downregulated. It was also shown that the level of IL-12 produced by PBMC of the patients stimulated with Df plus CpG-ODN was correlated positively with that of IFN-γ, and no cytotoxic effect on PBMC was demonstrated. In conclusion, CpG-ODN enhances the Th1 associated cytokines induced by Df mite allergen in both allergic and non-allergic individuals without significant cytotoxic effect, suggesting CpG-ODN as a new promising adjuvant and its potential application in the immunotherapy of allergic disorders.