Pattern recognition receptors of the innate immune system are able to distinguish certain prokaryotic DNAs from vertebrate DNAs by detecting unmethylated CpG dinucleotides in particular base contexts ('CpG motifs'). Recent studies have begun to define the molecular mechanisms of actions of CpG motifs and have demonstrated their stimulatory effects on leukocytes from humans and vertebrates other than mice. Oligodeoxynucleotides containing CpG motifs are highly effective Th1-like vaccine adjuvants through multiple routes of immunization and show promise as immunotherapeutic agents for cancer and allergic diseases.