A method of measuring the phagocytic activity of the leucocytes was developed by Leishman in 1902. In 1903 Wright and Douglas made an extensive study of the phagocytes, when brought in contact with suspensions of _Staphylococcus pyogenes_, and were able to show: (1) Quoting from the original: “We have here conclusive proof that the blood fluids modify the bacteria in a manner which renders them a ready prey to the phagocytes. We may speak of this as an ‘opsonic’ effect (_opsono_ [a Latin verb], I cater for; I prepare victuals for), and we may employ the term ‘opsonins’ to designate the elements in the blood fluids which produce this effect.” (2) Normal blood serum and plasma possess the same opsonic action upon bacteria. (3) Serum loses its opsonic action when heated to 60° C. This is spoken of as inactivated serum. (4) Inactivated serum and physiological salt solution have the same influence upon phagocytosis. (5) “The opsonic power of the blood fluids disappears gradually on standing,” losing about 50 per cent. of its activity in five or six days. (6) An anti-opsonic effect is noted when blood serum is digested with typhoid bacilli. (7) When a condition of immunity is conferred upon patients infected by Staphylococci, by vaccination with heated cultures of Staphylococci, the opsonic action of the patient’s blood serum is greatly augmented. Neufeld and Rimpau (1904) found in antistreptococcic and pneumococcic sera evidence of substances which, while inactive towards leucocytes, possessed very marked opsonic or sensitizing action, as they termed it, toward corresponding cultures of streptococci and pneumococci. Ross (_Lancet_, November, 1906) summarizes our knowledge of the opsonins as follows: “(1) Opsonins act by chemically uniting with the invading bacteria, and so altering them that the leucocytes are able to phagocyte the bacteria and destroy them. It is important to remember that these substances do not stimulate or otherwise affect the leucocytes. “(2) It is probable that there are present many varieties of opsonins in the blood plasma, each having to do with combating a particular kind of microbic invasion. “(3) Opsonins have been shown to be distinct from other bacteriotropic substances, such as the bacteriolysins, the agglutinins, and the antitoxins.” The leucocytes of healthy or diseased persons seem to be equally active when brought in contact with the same serum, hence the amount of opsonins present in the blood of an individual determines, according to the opsonic theory, his susceptibility to bacterial invasion. TECHNIQUE. To measure the resistance of the patient to such invasion, or to find out his opsonic index, special technique has been developed, which may be briefly described as follows: I. EMULSIONS OF BACTERIA.