Immunologic findings in the Obese Strain (OS) of chickens are described. This strain was obtained by selective breeding of individual OS chickens which comprised less than 1% of the original flock of the Cornell C-strain. At the present state of breeding about 80% to 90% of the individual chickens exhibit the OS characteristics. Tanned red blood cells from chicken, sheep and human coated with chicken thyroid extract are agglutinated by the majority of sera of OS chickens to various titers. Only chicken thyroid extract and no other chicken organ extract is capable of inhibiting the reaction. On the other hand, cells coated with human thyroid extract are essentially negative, showing the species-specificity of the chicken thyroglobulin antibodies. Precipitins can be easily demonstrated in the sera of OS chickens. By both methods (indirect hemagglutination and precipitation) the autoantibody nature of the thyroglobulin antibody can be demonstrated using thyroid material from the antibody-producing chicken itself. Upon sucrose gradient fractionation of OS chicken serum the highest hemagglutination titer appeared in the lighter antibody fraction and a lower one in the heavier antibody region.