Fifty strains of Haemophilus influenzae and 30 strains of H. parainfluenzae were clearly differentiated by the requirement of H. influenzae strains for haemin on a proteose-peptone basal medium and with an inoculum of 1-20 cells. No evidence was obtained for a wide range in haemin requirements; 10 strains of H. influenzae examined in detail required 2-10 pg haemin per ml. The two species were dissimilar also in the amount of NAD required; the strains of H. influenzae grew well in the presence of NAD 0·2-1·0pg per ml, whereas most strains of H.parainfluenzae required 1·0-5-0pg per ml and three strains required NAD 25 pg per ml for optimal colony size. Nitrate was essential for the anaerobic growth of H. parainfluenzae but not of H. influenzae.


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