IgA, IgG and IgM is faeces were quantified by single radial immunodiffusion using extracts of freeze-dried faeces. IgA in small specimens of faeces seemed to mirror the total amound of IgA secreted into the gut at the time of sampling. Presumptive normal values of faecal IgA concentrations in infants and children were established. Agglutinins to rabbit erythrocytes served as markers for the antibody activity. Infants and children just recovered from enteritis of probably infectious origin had higher concentrations of both IgA and agglutinins in faeces. Faeces from three out of five patients with ulcerative colitis in remission, contained IgG in markedly increased concentrations. Two patients with IgA deficiency had no detectable IgA in faeces, but had increased levels of faecal IgM which also agglutinated rabbit erythrocytes. Immunoglobulins were not demonstrated in faeces from three patients with agammaglobulinaemia. The findings indicate that faeces can be used for assay of immunoglobulins of the intestinal tract.