Precipitating factors against chicken serum proteins detected by double diffusion in agar gel were observed in 40 % of D.S. patients studied. The precipitating factors were found to be associated with γG globulins. Similar factors were absent in groups of normal children, mentally retarded subjects, and parents and siblings of the D.S. patients. No association was found between the precipitating factors and the presence of Pe antibodies or Au‐SH antigens.