1. Azoprotein antigens containing glucuronic and galacturonic acids give rise in rabbits to specific antibodies. The immune sera show no serological crossing with antigens containing glucose or galactose.
2. The galacturonic acid antigen reacts in antipneumococcus horse serum Type I in high dilutions.
3. Azoprotein antigens containing galacturonic acid, benzene sulfonic and carboxylic acids precipitate in antipneumococcus horse sera of various types but not in normal horse serum. The mechanism underlying these cross reactions is discussed.