A crude extract of the proventriculus of the Japanese quail gave at least five bands of peptic activity at pH 2.2 on polyacrylamide gel electrophoresis. The main component, constituting about 40% of the total acid protease activity, was purified to homogeneity by hydroxyapatite and DEAE-Sepharose column chromatographies. At below pH 4.0, the pepsinogen was converted to a pepsin, which had the same electrophoretic mobility as one of the five bands of peptic activity present in the crude extract. The molecular weights of the pepsinogen and the pepsin were 40 000 and 36 000, respectively. Quail pepsin was stable in alkali up to pH 8.5. The optimal pH of the pepsin on hemoglobin was pH 3.0. The pepsin had about half the milk-clotting activity of purified porcine pepsin, but the pepsinogen itself had no activity. The hydrolytic activity of quail pepsin on N-acetyl-L-phenylalanyl-3,5-diiodo-L-tyrosine was about 1% of that of porcine pepsin. Among the various protease inhibitors tested, only pepstatin inhibited the proteolytic activity of the pepsin. The amino acid composition of quail pepsinogen was found to be rather similar to that of chick pepsinogen C, and these two pepsinogens possessed common antigenicity.