The causative agent of smallpox is not positively known, but many facts about its nature have been learned, and the effect of its action on the epidermis has been carefully observed. In former years, many bacteria isolated in smallpox cases were described as the cause of this disease. Today, however, the presence of these bacteria is looked on as accidental, while the contents of variola vesicles are usually free from bacteria. The disease can be transmitted with inoculations of this clear, bacteria-free fluid, and passage through a Berkefeld filter does not destroy the potency. In the severe forms of smallpox, streptococci have been found frequently in lesions and in the blood stream. The investigations of Hines1 in the Chicago epidemic of 1922, using the modern grouping, showed the presence of both hemolytic and nonhemolytic streptococci. Arndt2 in a careful study of the Dresden epidemic of 1918-1919 found streptococci