We examined the anti-tumour activity of exosomes derived from dendritic cells (DCs) in combination with cyclophosphamide (CTX) and polyinosinic-polycytidylic acid sodium salt (poly I:C). DCs were pulsed with L1210 lymphocytic leukaemia cell antigen and lipopolysaccharide. The exosomes that the DCs secreted were purified. In vitro, the anti-tumour activity of exosomes was assessed by measuring their ability to induce spleen cell proliferation and the extent to which they induced spleen cells to kill L1210 cells. Poly I:C was able to induce DC maturation. DC-derived exosomes stimulated spleen cell proliferation and enhanced the cytotoxic effects of spleen cells in vitro. DC-derived exosomes, in combination with CTX and poly I:C, suppressed L1210 tumour growth in vivo and gave the greatest prolongation of survival time in tumour-bearing DBA2 mice. These findings suggest that this combination of a tumour vaccine, a conventional anti-cancer agent and a promoter of DC maturation might be a useful anti-cancer therapy.