1. 19 strains of non-hemolytic hemophilic bacilli were studied. 17 required the addition of V and X growth accessory factors to their media, 2 required the addition of only V.
2. Of 15 strains of hemolytic hemophilic bacilli examined, 10 were found to require the addition of only V to their media, 3 the addition of V and X in the form of yeast extract and hematin, and 2 the addition of accessory growth factors in the form of blood.
3. In media to which neither V nor X had been added true symbiosis was found to occur on growing B. hemoglobinophilus canis, requiring the addition of X, with B. parainfluenzæ, requiring the addition of V, or with hemolytic strains of bacilli, requiring the addition of only V.