The use of symbiotic bacteria has been extensively studied in investigations of the biology of B. influenzæ, and it has been shown, as noted elsewhere, that such accessory bacteria will bring to growth B. influenzæ on media otherwise quite unsuited to its needs. It has been further found that on various preparations of hematin agar, on which B. influenzæ refused to grow, such media could be rendered favorable for their growth by the addition of living or freshly killed cultures of staphylococcus and many other bacteria. And although the method is well known, it has not been extensively used for the purposes of isolation. Many of the workers, however, have pointed out the importance of looking for growth of the influenza bacillus in the neighborhood of the more easily grown bacteria which almost always develop in cultures from the respiratory tract. Grassberger has particularly studied this problem and has made practical application of the method. Accidental contamination of plates with air bacteria have made possible, in some instances, the isolation of B. influenzæ—as, for example, in the finding of Heyrovsky from a case of empyema of the gall bladder—while other workers have pointed out the difficulty of demonstrating growth where B. influenzæ is pure in the material cultured, and the comparative ease and relative luxuriance of growth where other bacteria are present. To just what this stimulating effect is due has been much discussed, and it is generally agreed that the hemoglobin is markedly changed and rendered more available by the action of these germs. It is to be noted that on a medium containing blood altered by heating or by the various methods as described by Fleming the foreign bacteria no longer show any symbiotic action on B. influenzæ. Grassberger considered the effect of the bacteria on the blood to be the same as that of heating. Allen laid particular stress on this symbiotic character. He used a staphylococcus, either living or killed, in making his cultures and noted the difficulty of growing B. influenzæ from material in which it occurred pure. W. F. Robertson made use of these facts of symbiosis for both isolation and stimulation of growth. He employed alternate drills of M. catarrhalis or pneumococcus with the B. influenzæ, and Brown and Orcutt used strains of hemolytic streptococci for the same purpose. The latter authors considered that the beneficial effect of the streptococci was merely due to the setting free of the hemoglobin. The fact that similar results are to be obtained by the use of non-hemolytic bacteria as well as forms giving green color changes to the blood makes this explanation untenable. In my own studies I have confirmed the results of several previous workers. I have found that B. influenzæ is stimulated in its growth by the presence near it of colonies of staphylococcus pyogenes aureus and albus, pneumococci, streptococcus viridans and hemolyticus and other bacteria.