5,7,3',4'-Tetrahydroxyflav-2-en-3-ol 3-O-β-D-glucoside was isolated from the seed coats of immature black soybeans (Glycine max (L.) Merr.). This compound is a reduced form of cyanidin 3-O-β-D-glucoside (cyanidin 3-G) which was obtained by reaction with hydrochloric acid. The molecule has reducing activity for a tetrazolium derivative (WST-1) in the presence of 1-methoxy-5-methylphenazinium methylsulfate (1-methoxy PMS) in a similar manner to NADH. The seed coats of immature black soybeans also contain epicatechin as a major constituent, while cyanidin 3-G and procyanidin B2 are present at lower concentrations. Immature brown soybeans did not contain 5,7,3',4'-tetrahydroxyflav-2-en-3-ol 3-O-β-D-glucoside, but did contain both epicatechin and procyanidin B2. Immature yellow soybeans contained none of them.