"Delicious peptide" was reported to be as delicious as beef soup. The peptide and its fragments were synthesized to study its taste active sites. "Delicious peptide" was found to produce a umami and a sour taste. By preparing several di- or tripeptides composed of basic or acidic amino acids, we found that the taste of "delicious peptide" was produced by an interaction between the basic and the acidic fragments in the peptide.