Deoxyribonucleic acid (DNA) of bacteria (Escherichia coli, Pseudomonas aeruginosa, and Bacillus subtilis) repressed the growth of yeasts (Saccharomyces cerevisiae, Hansenula anomala, and Schizosaccharomyces pombe) and a fungus (Aspergillus niger), whereas the DNA of S. cerevisiae did not significantly repress the growth of bacteria. Chemically synthesized single-stranded oligonucleotides with the CpG dinucleotide motif also repressed the growth of S. cerevisiae. The effect of E. coli DNA was partially abolished after complete depolymerization or methylation of the DNA, and that of the oligonucleotides with the CpG motif decreased when the cytosine in the motif was methylated. The observed repression of eukaryotic microbial growth by bacterial DNA is thought to be due to the presence of the CpG motif. The results indicate that DNAs have some physiological function in addition to being carriers of genetic information.