CG sequences (CpG ODNs) can activate innate immune and thus be potentially useful for immune stimulants in therapeutic use. By hybridizing a CpG ODN having a (dA)(40) tail with poly(dT), we prepared a DNA duplex that multiple CpG ODNs are hanging from the duplex. IL-12 cytokine secretion reached the maximum at the composition that two to four CpG ODNs in one duplex. These phenomena can be rationalized by an allosteric or cluster effect, and the present clustered CpG system is the simplest and most elucidating tool to investigate the cluster effect of CpG ODNs.