(1) Inoculation of pathologically active crusts on guinea-pigs caused no reaction, and in this respect a marked difference was demonstrated clinically between the disease of which the crusts were a product, and vaccinia. (2) On calves the disease tended to show some vaccino-variolal relationship, from the appearance of its vesicles, but it appeared to be distinct on the one hand from vaccinia, and from variola on the other. (3) Calf experiments, two only in number, suggested that vaccinia and variola failed to protect against Australian disease, and that Australian disease failed to protect against vaccinia or variola. (4) On monkeys the vesicles of Australian disease were undistinguishable in appearance from those of vaccinia or variola, but the evidence for immunity relationship between Australian disease and vaccinia and variola was of the slightest; Australian disease affording no protection against vaccinia or variola, vaccinia possibly protecting against Australian disease in 25 per cent, of the cases, and variola possibly protecting against Australian disease in 40 per cent, of the cases. Vaccinia protected against variola in 100 per cent, of the cases, and variola against vaccinia in 50 per cent, of the cases.