Summary: The applicability of the hemagglutination and hemagglutination-inhibition methods to the solution of variety of immunological problems has been described. These include the analysis of anti-protein sera of varying specificity from different species, the determination of the heterogeneity of antigens and antibodies, the analysis of proteins in mixtures of proteins, the detection of non-precipitating or poorly avid antibodies, and the analysis of supernatants of antigenantibody mixtures. It was shown that egg albumin was adsorbed about as well as conalbumin to tannic acid-treated red blood cells but that the egg albumin-anti egg albumin system was much less sensitive than the conalbuminanti conalbumin system for the detection of antibody and antigen. The advantages and disadvantages of the great sensitivity of these methods was discussed. The apparent ability of the reactions to detect non-precipitating and poorly avid antitoxins was also considered. Possible explanations of the lack of correlation between hemagglutination titer and unitage of human and guinea pig antitoxins and the good correlation between these figures for rabbit antitoxins were given.