The concept of immunostimulatory DNA was borne in a long series of studies on BCG-mediated tumor resistance. DNA purified from BCG inhibited the growth of various syngeneic animal tumors, augmented NK cell activity and induced IFN-alpha/beta and -gamma from mouse spleen cells and human PBL. Extending the lines of study, we found two biologically remarkable facts that (i) DNAs from invertebrates, but not from vertebrates and plants, showed the above-mentioned biologic activities, and (ii) the activities were completely dependent on particular base sequences having CpG motifs but in a senseless manner. Details of those early studies carried out mainly in the 1980's have been reviewed in the first part of this paper. In the middle part of this review, the results of toxicity and pharmacology studies and clinical trials of BCG- DNA, performed by other groups in Japan in the late 1980's, were introduced. Since a large amount of DNA had never been administered repeatedly into experimental animals or human, those experiences obtained seem to be worthwhile to introduce. Research interests of immunostimulatory DNA were galvanized in 1995 by the report of Krieg et al. showing murine B cell activation with bacterial DNA containing CpG motifs. Within a short period of time, a huge number of papers have been published in this field, and the study has expanded rapidly and largely. Now, it includes a number of research fields, for example, host-defense mechanisms against infection, allergy, autoimmune diseases, cytokine networks, plasmid vaccination, and therapeutic application of certain diseases. This paper reviewed briefly recent advances of immunostimulatory DNA research. The response of higher animals against immunostimulatory DNA must be the most primitive but important mechanism for self-nonself discrimination against foreign DNA. By utilizing immunostimulatory DNA or controlling this primitive response, it seems possible to offer many beneficial means to human health. For instance, more potent peptide- or plasmid- vaccines could be developed by the use of immunostimulatory DNA. On the other hand, many study results suggest that immunostimulatory DNA works either beneficially or harmfully for the hosts. We assume that further extensive and careful studies are required.