Abstract
Many rheumatoid sera markedly enhance the agglutination of sheep cells sensitized by a non-agglutinating amount of rabbit antibody, whereas most normal sera do not demonstrate such agglutination enhancement. The nature of the factor in rheumatoid serum responsible for the agglutination enhancement has not been defined. The evidence accumulated at present is compatible with this factor being either an antibody to a normal constituent of the gamma globulins or a component of complement.