Interleukin (IL)-23, a cytokine composed of p19 and the p40 subunit of IL-12, can enhance the proliferation of memory T cells and production of IFN-gamma from activated T cells. It can also induce antitumor effects in murine model. To further evaluate the antitumor activity and immune enhancement of IL-23 in vivo, murine colon carcinoma cells retrovirally transduced with mIL-23 gene were injected subcutaneously (s.c.) into BALB/c mice. Survival time and tumor volume were observed. LDH release assay, [3H]-TdR incorporation assay and ELISA were used to determine CTL activity, proliferation of splenocytes and level of cytokines, respectively. Number of dendritic cells (DCs) was analyzed by flow cytometry (FCM). IL-23 secreted by Colon26/IL-23 cells suppressed the growth of tumor and prolonged the survival time of mice, enhanced proliferation of splenocytes, CTL activity, and number of DCs. IL-23 also promoted the production of Th1 cytokines such as IFN-gamma, IL-12 and TNF-alpha. However, the level of IL-4 was not enhanced significantly. These data suggested that IL-23 secreted by tumor cells can induce antitumor activity by enhancing immune response.