The fragment of the Fc segment of IgE resistant to proteolytic enzymes was determined by a radioimmunologic method in fecal extracts from several groups of patients. IgEs remained undetectable in the 95 healthy subjects studied. IgEs were detected in 16/27 carriers of intestinal parasites (60%), with a mean of 92.4 IU/g dry weight. IgEs were also detected in 236/312 food-sensitization patients, with the sensitizing foodstuffs being identified by searching for the specific IgEs in circulation (75%), with a mean of 63.9 IU/g dry weight. The simultaneous determination of alpha-1-antitrypsin in fecal extracts and in sera from 21 control subjects and 21 food-sensitization carriers demonstrated that extravasation of plasma proteins into the gut lumen cannot be responsible for the presence of IgE in stools. Testing for IgE in stools therefore appears to be a simple and economical means of identifying patients without parasites who present with food sensitization.