Several examples have shown that C3d3, when fused to a corresponding antigen, had a strong adjuvant effect on certain specific antibody production. In a previous study, we attempted to prove that this was the case of the human chorionic gonadotrophin β chain (hCGβ)-induced immunity following DNA vaccination. However, we found that C3d3 when fused to hCGβ inhibited rather than enhanced the antigen-specific immune response. In the present study, using hCGβ DNA vaccine preparations, we demonstrated that C3d3 inhibited the antigen-specific humoral antibody response and several other immune responses, such as the hCGβ specific lymphoproliferation. Such inhibitory effects of C3d3 were not related to the expression level of the target protein, the gender of the test mice, or the vector used. Contrastingly, C3d3 fused with the envelope protein of hepatitis B virus (PreS2/S) used as a control system resulted in the enhancement of both humoral and cell-mediated antigen-specific immune responses against HBV-preS2/S, which was consistent with other groups' adjuvant-effect findings. We further showed that the mechanisms involved in the inhibitory effect of C3d3 might be possible due to impairing the function of antigen presenting B lymphocytes and reducing the expression of transcription factors (T-bet and GATA-3) and cytokine IL-4. Collectively, unlike its usual expected adjuvant function, the fusion of C3d3 with the tumor-associated antigen hCGβ was found to inhibit both humoral and cell-mediated antigen-specific immune responses. These findings indicate that research concerning tumor immune escapes and vaccine designs require further extensive attention. © 2006 Springer-Verlag.