It has been shown that in the serum of persons convalescent from infectious diseases or vaccinated (by bacterins) against certain infectious diseases substances are present which prepare the micro-organisms for the action of the phagocytes. These substances are termed opsonins. If fresh blood is mixed with an emulsion of some bacteria, and then incubated for one-half hour, it will be found that many of the bacteria are within the polymorphonuclear leukocytes. If the serum is washed away from the leukocytes before adding the bacteria, none of the latter will be found within the leukocytes. In order to show that this effect is on the bacteria rather than on the leukocytes, the bacterial suspension may be treated with some serum for one-half hour, then washed free from this serum by means of salt solution and a centrifuge, and then mixed with some serum-free leukocytes; then it will be found that phagocytosis occurred as before.

The estimation of the opsonic power of the serum, has been attempted and various methods elaborated. It may be questioned, however, whether any of the tests now in use is a true index of the amount of opsonins in the serum, although it may be taken to indicate roughly the measure of their activity.