It was shown in a preliminary report (1) that the filtered blood broth cultures of B. influenzae are toxic for rabbits. It was stated that the serums of rabbits which had been immunized to the poison possess neutralizing power for the poison. Whether this poison belonged to any known class of poisons was not established. It is the purpose of this paper to give in greater detail the work done to date on this poison of the influenza bacillus with especial regard to its possible classification. Experimental Preparation of media. The poison is produced by the growth of B. influenzae on several different mediums, in fact, on any medium which contained rabbit hemoglobin in sufficient quantities. The medium, however, from which the poison for this work was obtained, was made as follows.