In the course of an investigation, made for the purpose of determining whether the cells of various tissues can absorb the isohemagglutinins with the same degree of specificity as the erythrocytes, the very remarkable fact has been discovered in my laboratory by S. Shirai that human semen manifests a very marked inhibiting power upon isohemoagglutination, and, moreover, there is a fair degree of specificity in this inhibition. Just in the like manner as the red blood cells of group I blood (Jansky) do not absorb the isohemoagglutinins, and consequently can not alter the agglutinating power of any serum, the semen produced by a man of group I blood has either no influence upon the agglutinating power of serum of any blood group, or its inhibiting influence is impartial upon all types of sera, if some inhibition occurs.