A score and more kinds of molluscan shells have been used as drugs in oriental medicine. The Korean Pharmacopoeia (K.P.) contains a monograph on oyster shell "Ostreae Testa" as an official drug. A huge deposit of the fossil shell Crassostrea gravitesta eoilensis has been recently excavated in the region of Wolsung, Kyong-Sang-Buk-Do. This study was conducted to determine the applicability of the fossil shell as a substiute for Ostreae Testa, K.P. The fossil shell not only met the K.P. requirements, but also exhibited an appropriate antacid activity. Amino acid survivals were also determined quantitatively with amino acid autoanalyzer.