The susceptibility of exocrine and serum immunoglobulins and antibodies to proteolytic degradation was assessed. Colostral and duodenal fluid exocrine 11S IgA, monomeric serum IgA, and IgG were digested with trypsin, chymotrypsin, or duodenal fluid. Exocrine IgA was more resistant to digestion than were the serum immunoglobulins. Under conditions of the experiments, most of colostral IgA retained its 11S quaternary structure, including the secretory piece; the portion degraded was reduced almost entirely to peptides. The superior resistance of exocrine IgA was also demonstrated by digestion of serum IgG and nasal exocrine IgA diphtheria antitoxins with trypsin or duodenal fluid. Selective precipitation of trypsin-digested antitoxins with antibodies to heavy chains, light chains, or secretory piece revealed that the differences in susceptibility to digestion were due to differences in lability of the Fc portions of the IgA and IgG antibody molecules. The Fc portions of IgG antibody molecules were degraded or cleaved from the Fab units of the molecules, whereas the Fc-like portions of IgA antibody molecules remained associated with their Fab-like units and the secretory piece. On the other hand, trypsin treatment did not affect the antigen binding ability of the Fab parts of either the exocrine IgA or IgG antibodies. The Fc-like portions of exocrine IgA may be protected from tryptic degradation by the quaternary structure of the 11S molecules, which includes a dimer of 7S IgA subunits and the secretory piece.