Dendritic cells (DC) play a central role in the initiation and regulation of immune responses. Increasing evidence has indicated that manipulation of DC can serve as a therapeutic mechanism for immunomodulation. In this study we tested some unique compounds isolated from Antrodia cinnamomea, a medicinal fungus in Taiwan, on mouse bone marrow-derived DC activation. A triterpenoid methyl antcinate K (me-AntK) promoted DC maturation by enhancing the expression of MHC class II, CD86, and reducing the endocytosis. TNF-alpha, MCP-1, and MIP-1beta were secreted by DC after me-AntK treatment, indicating augmentation of innate immunity by me-AntK. Interestingly, the me-AntK-activated DC induced Ag-specific T-cell proliferation and facilitated Th2 differentiation. Examining signaling responses, we found that me-AntK treatment uniquely activated JNK and ERK in DC. Our results demonstrate that me-AntK is the first natural triterpenoid to promote the ability of DC to prime Th2 responses. This suggests that me-AntK can potentially be applied to enhance immune responses and modulate DC function in immunotherapy.