The present study was conducted to find an immunostimulatory oligonucleotide derived from yogurt starter cultures. The chromosomal DNA was purified from nine strains of Lactobacillus delbrueckii ssp. bulgaricus and six strains of Streptococcus thermophilus. An immunostimulatory ability of the DNA was examined in a proliferation of peyer's patch and splenic B cells. Only the DNA from L. bulgaricus NIAI B6 induced a significant proliferation of both cells. When the DNA was cloned and amplified using PCR, the mitogenic activities to B cells were significantly increased by 13 of 135 DNA clones. Ten homologous nucleotide sequences were found as possible oligonucleotide sequences of mitogens, and were then chemically synthesized (sOL-LB1 to sOL-LB10). One CpG-like motif (sOL-LB7; 5'-CGGCACGCTCACGATTCTTG-3') was identified as an immunostimulatory oligonucleotide, but it did not contain palindromic CpG structure known as a B cell-specific mitogen. The sOL-LB7 substantially bound to B cells and increased the CD69 positive cells in peyer's patch cells. This study demonstrated that L. bulgaricus NIAI B6 was a good candidate of a starter culture for the production of new functional foods, "Bio-Defense Foods".