To determine whether RNA interference (RNAi) could block cytotoxic T-lymphocyte antigen 4 (CTLA4) in human lymphocytes in vitro and promote IFN-gamma and IL-2 secretions, three small interfering RNAs (siRNAs) were selected based on target specificity sequences of human CTLA4 and transfected into human lymphocytes of chronic HBV patients. As a result, the expression of human CTLA4 mRNA was efficiently suppressed by all the three siRNAs. Compared with negative control (siRNA-co), siRNA-1 inhibited the expression of CTLA4 most efficiently and was used in the further study. The expressions of IFN-gamma and IL-2 were upregulated and the level of IL-4 was almost unchanged in lymphocytes transfected with siRNA-1 compared with the blank control. These results indicated that siRNA-1 led to IFN-gamma and IL-2 secretions, which is a main response of Th1/Th2. In a conclusion, RNAi significantly suppressed the expression of human CTLA4 mRNA in human lymphocytes in vitro, and could induce Th1/Th2 response. It could be a new therapeutic strategy for chronic HBV infection.