Summary and Conclusions: Diluting a hemagglutinating serum with an antagonistic or previously absorbed hemagglutinating serum instead of with saline solution diminishes the titre of agglutination considerably, in some cases to zero. Since this result can be achieved by any human serum freed of agglutinins, even by sera of the same type and by O serum, therefore it cannot be considered as caused by the type-specific substances (agglutinogens), but must be caused by an unspecific factor which can be found in different degree in any human serum. Heating to 56 C for one-half hour does not affect this factor.