Microscopic study of the lungs of treated animals sacrificed at 6, 18, 42, 66, 96, and 168 hours after the start of treatment revealed the following sequence of events. During the first 18 hours the drug apparently had little effect upon the pneumonic lesion, but at the end of 18 hours pneumococci in the edema zone began to show striking changes in their morphology, indicating bacteriostesis. Forty-two hours after the start of treatment the edema zone had disappeared, the pneumonia had ceased to spread, and the pneumococci at the margin of the lesion had been overtaken by leucocytes. Careful examination of the exudate in the periphery of the lesion revealed definite phagocytosis of pneumococci. By the 4th day no pneumococci could be found in the stained sections, and after 1 week there remained only macrophages in the rapidly clearing alveoli.

In order to demonstrate the phagocytic reaction more clearly the effect of sulfonamide drugs was studied in pneumonic rats previously rendered leucopenic by exposure to x-ray. The pneumonia in these animals was relatively acellular, and the few macrophages present in each alveolus could be seen to have phagocyted large numbers of pneumococci after 18 to 42 hours of treatment. The macrophages not only phagocyted the pneumococci but ultimately destroyed them, the pneumonic lesion later going on to complete resolution. The fact that this phagocytic reaction was observed in the lungs of animals with bacteremia suggests that the phagocytosis is independent of circulating type-specific opsonins.