Experiments recently reported (18) have been interpreted to indicate that surface phagocytosis plays no significant part in natural antipneumococcal defense. A repetition of these experiments has revealed: (a) that the cellular content of the leucocytic suspensions used in the phagocytic tests was of a different order of magnitude from that of the exudates which usually exist in infected tissues, (b) that the suspensions were too dilute to allow surface phagocytosis of pneumococci to occur, and (c) that the ratio of bacteria to leucocytes. was such that, when a sufficiently concentrated exudate was employed, the pneumococci injured the leucocytes and thus prevented phagocytosis from taking place.
When conditions of the tests were suitably controlled, and conventional quantitative methods were employed to measure the end results of the phagocytic reaction, the essential observations relating to surface phagocytosis were fully confirmed. The significance of this non-antibody mechanism of defense in pneumococcal infections was thus further substantiated.