A fragment of IgE with molecular weight of about 40,000 was identified by radioimmunoassay in human small intestinal fluid after fractionation by gel filtration chromatography. Digestion of E myeloma protein PS by pooled intestinal fluid, trypsin or chymotrypsin yielded a degradation product of similar molecular weight that probably consisted principally of ε1determinant-containing fragments. These findings suggest that IgE is secreted into the intestinal lumen and degraded there by pancreatic proteolytic enzymes, producing an enzyme-resistant portion of the amino-terminal part of the Fc region.