Antibody activity in feces was investigated by testing for agglutinins to rabbit erythrocytes Extracts of freeze‐dried feces from 13 of 14 infants and children agglutinated the rabbit erythrocytes, as did the sera from ill all these individuals. Feces from I‐month‐old infants gave relatively high titres, whereas sera gave loss titres compared with what was found in older children. Thus, the intestinal agglutinins seemed to be produced independently of those in serum. The fecal agglutinins correlated well with the concentration of IgA in feces and were inhibited with antiseri In IgA. indicating that IgA in feces was mainly responsible for the agglutination Results of preparative ultracentrifugation and gel filtration as well as immunization studies were in line with this There is thus evidence that only IgA retains its, activity in the gut. even when IgG and IgM are present in feces. Since agglutinins to rabbit erythrocytes in serum are mainly confined to IgG and IgM, this activity in feces seems to be a useful market for locally produced intestina