The discovery by Pfeiffer of the hemophilic character of the bacillus found by him in cases of influenza opened up a new group of micro-organisms known as the hemophilic bacteria. Davis (1915) has laid particular stress on the group character of these bacilli, and the more they are studied the more clear does it become that there are several distinct members. The B. influenzæ is by far the most important as well as the most frequently found of the group and is considered as the type organism. All these bacteria require for their growth the presence of some form of hemoglobin. The actual amount necessary may be very small, and Davis suggested that it may have a catalytic action. A great deal of work has been done in attempts to discover just what portions of the hemoglobin are necessary to bring about this phenomenon. In our discussion on media for the influenza bacillus we will briefly describe some of the various hemoglobin preparations that have been used successfully. It must at this point be emphasized that blood is very useful in many media to stimulate the growth of a great variety of bacteria, and the transfers made from such luxuriantly growing cultures may grow very poorly or not at all on ordinary media, and this might easily lead to erroneous conclusions on the hemophilic character of the organisms studied. There are certain bacteria which grow so much better on media containing blood that such media are sometimes necessary for their isolation, although after a few transfers they will grow on ordinary media. This is true for bacillus pertussis, and throughout the literature a good deal of confusion has arisen in not recognizing this temporary hemophilic character of certain bacteria. The true hemophilic bacteria do not grow except in the presence of hemoglobin in some form or other. The problem becomes almost academic when we consider the small amounts of hemoglobin that are necessary. Davis has shown that a dilution of 1 in 180,000 is sufficient, and in the interesting discussion between Cantani and Ghon and Preyss it was demonstrated that hematin or other hemoglobin product was necessary in the agar before B. influenzæ would grow in the presence of other bacteria, and that this hematin could be derived from the blood in the meat which was used in making the basic infusion.