Enzymatic proteolysis of human immunoglobulin A yields Fabα fragments but the Fcα is degraded to small peptides. The unavailability of Fcα has hampered to some extent certain studies of IgA, e.g., its primary structure and the attachment site of secretory component and J chain. Recently, while studying human intestinal immunoglobulins, it was observed by Mehta et al. (1) that normal feces contain an IgA fragment which was shown to be Fcα, and that feces contain a proteolytic enzyme which cleaves both serum and secretory IgA to yield Fabα and Fcα. Of considerable interest is that the enzyme is produced by bacteria in those regions of the alimentary tract heavily populated by bacteria, principally the mouth and the colon. We have termed the enzyme IgA protease because enzymatic activity has thus far been found only against IgA.