Agglutination tests have been used by many investigators in attempts to determine a specific reaction in the sera of persons suffering from influenza. Such reactions develop, as we know, against secondary infecting bacteria, so that unqualified conclusions cannot be drawn that agglutinins in the sera of patients against B. influenzæ indicate the etiological importance of this organism. Vagedes using a dilution of 1-50 found 8 positives among 27 patients tested. Lord found the test most inconstant. Ghedini obtained useful results by using serum in dilutions 1-20 to 1-30, and had 17 positives from 28 influenza cases. He found agglutinins present three to four days after the height of the infection, and noted that the sera became practically normal after three to four weeks. Fichtner, although he obtained agglutination with sera of influenza patients in high dilutions (1-100 and 1-750), found his controls were often agglutinated, and consequently drew no conclusions. Wollstein (1906) did a series of agglutination tests, using various strains of B. influenzæ. The sera of patients she found very unsatisfactory, but by immunizing rabbits with this organism she obtained sera with titres up to 1 in 400. She could find no differences among the various strains studied. Somewhat similar results were obtained by her in 1915 working with strains from the meninges and the respiratory tract. Odaira carried out a rather extensive series of tests, using immunized rabbit sera and a special method of making his bacterial emulsions. He was able to distinguish B. influenzæ from both B. pertussis and the so-called Cohen’s bacillus of meningitis. Friedberger’s dog bacillus, however, could not be differentiated from B. influenzæ by this means. A. Fleming during the recent epidemic had good results with the sera of 21 patients. He incubated at 50° C. for two hours. He also used sera of immunized rabbits and got marked agglutination against the homologous strain, but varying results with other strains. He noted some strains agglutinated readily, while others did not. Eyre and Lowe noted an increase in agglutinins in the sera of people vaccinated against the influenza bacillus. Couret and Herbert could distinguish two types and a possible third among their strains. Park and his co-workers found numerous types by means of agglutination. Absorption of agglutinins was found helpful by these last two workers. There are so many factors capable of altering the sensitiveness of bacteria to agglutination, as in the well-known experiments of Neufeld, that we must recognize that much work is still to be done before we can properly interpret the results of these agglutination tests. _Binding of Complement_