ABSTRACT: Koikuchi and tamari shoyu are 2 types of traditional Japanese soy sauce whose low-molecular-weight fractions (less than 500 Da) are known to have the most intense umami taste. Given that peptides were also abundant in some umami fractions of both shoyu, further investigation was conducted to evaluate the precise contribution of peptides to the soy sauce tastes. To locate and isolate peptide fractions, separation procedures using Sephadex G-25 SF, Sephadex G-10, and reversed-phase high-performance liquid chromatography (RP-HPLC) linked to sensory evaluation (by taste dilution analysis), as well as amino acid analysis were performed. Peptides having glutamyl residue were present in most peptide fractions. However, they all revealed negligible contributions to the intense umami taste of the soy sauces. On the other hand, free Glu, in the presence of sodium salt, as well as free Asp and several sweet-taste eliciting free amino acids were considered to be the main contributors to the umami taste of the soy sauces. This study of Japanese soy sauces gives a clear explanation of the frequently questioned peptide contribution to the taste of savory food product.