The chemical nature of opsonins is not certainly determined, but they appear to be a distinct class of antibodies and to possess two groups, a combining or haptophore and a preparing or opsonic group and hence are similar to antibodies of Ehrlich’s second order--agglutinins and precipitins. Wright also showed that opsonins are just as specific as agglutinins are--that is, a micrococcus opsonin prepares micrococci only for phagocytosis and not streptococci or any other bacteria. Wright showed that opsonins for many bacteria are present in normal serum and that in the serum of an animal which has been immunized against such bacteria the opsonins are _increased_ in amount. Also that in a person infected with certain bacteria the opsonins are either increased or diminished, depending on whether the progress of the infection is favorable or unfavorable. The _opsonic power_ of a serum normal or otherwise is determined by mixing an emulsion of fresh leukocytes in normal saline solution with a suspension of the bacteria and with the serum to be tested. The leukocytes must first be washed in several changes of normal salt solution to free them from any adherent plasma or serum. The mixture is incubated for about fifteen minutes and then slides are made, stained with a good differential blood stain, Wright’s or other, and the average number of bacteria taken up by at least fifty phagocytes taken in order in a field is determined by counting under the microscope. The number so obtained Wright calls the _phagocytic index_ of the serum tested. The phagocytic index of a given serum divided by the phagocytic index of a normal serum gives the _opsonic index_ of the serum tested. Assuming the normal opsonic index to be 1, Wright asserts that in healthy individuals the range should be not more than from 0.8 to 1.2, and that an index below 0.8 may show a great susceptibility for the organism tested, infection with the given organism if derived from the individual, or improper dosage in case attempts have been made to immunize by using killed cultures, vaccines, of the organism. On the occasion of the author’s visit to Wright’s clinic (1911) he stated that he used the determination of the _opsonic index_ chiefly as a _guide to the dosage_ in the use of vaccines.