The contribution of secretory component to the stability of secretory IgA against proteolysis has been studied by a new approach, i.e., by comparing the proteolytic degradation of the complexes formed in vitro between these proteins and secretory component. The results show that attachment of secretory components to the dimeric backbone of the secretory IgA molecule is accompanied by a significantly increased resistance of this backbone against digestion by both trypsin and pepsin. This protective effect may be a physiologic function of secretory component or may be due merely to unspecific blocking by secretory component of one or more sensitive peptide bonds in the IgA backbone.