Fab‐fragments of IgG were easily demonstrated in extracts of faeces from healthy infants and children. Employing immunoelectrophoresis with antisera to whole human serum and to Fab‐fragments of IgG, a marked precipitation line, likely to represent such fragments, was evident in the cathodal or γ‐region. Usually no precipitate was formed with antisera specific for γ‐heavy chains or Fc‐fragments of IgG. Presumably IgG in the gut is partially destroyed before being excreted with faeces. Results of immunoelectrophoresis, gel filtration as well as polyacrylamide gel electrophoresis, using antisera to a‐heavy chains, indicated the presence of fragments of IgA in some faecal extracts. Fragments of IgA could not be demonstrated in IgA‐deficient patients, and no fragments of either class, or only traces of Fab‐fragments of IgG, were found in agammaglobulinaemic patients. No antibody activity against rabbit erythrocytes was found in gel filtration fractions containing such fragments; any reaction was not observed either in the direct agglutination test or in a modified