The molecular size reduction and the formation of bitterness during a tryptic hydrolysis of soybean 11S glycinin were determined by using quantitative analysis and organoleptic evaluation. The 11S glycinin of 90% purity was prepared by cryoprecipitation and Con A Sepharose 4B affinity chromatography, and hydrolyzed with trypsin in a pH-stat reactor for 4 h. Bitterness was formed within 1 h of hydrolysis, and then slowly increased up to $3.5 imes10^{-5}$ M quinine-HCl equivalent. The extent of hydrolysis (DH) was 7% at 1 h and increased up to 12% by the end of the reaction. The -amino nitrogen content increased from an initial 0.7 mM to 7 mM at the end of the period. The SDS-PAGE analysis showed that the acidic subunit of 11S glycinin was mostly hydrolyzed. The GP-HPLC analysis indicated that the bitterness was mainly contributed by the peptide fractions of molecular weights of 360-2,100 Da.