To obtain information on the luminal absorption of oyster zinc, the zinc action during an in vitro protease digestion of oysters was examined. More than 90% of the zinc rendered solute at pH 1.3 or 3.0 irrespective of the pepsin digestion. The solute zinc was partially re-precipitated by neutralization, and trypsin digestion did not render the re-precipitated zinc solute. When the pepsin digestion was performed at pH 5.0, the ratio of soluble zinc in trypsin digest decreased. When the trypsin digest was fractionated by Sephadex G-25 , the zinc was eluted later than the peptide fragments. These results indicate that the pH of the stomach juice rather than peptides released by the digestion of oyster protein highly contributes to the oyster zinc in the small intestine becoming solute.