Abstract
1. 5 per cent sheep blood agar is a selective medium for beta hemolytic streptococci in throat cultures since sheep blood inhibits the growth of bacillus X (H. hemolyticus) and B. parainfluenzae hemolyticus. The growth of H. influenzae is also inhibited by sheep blood. 2. The inhibitory action of sheep blood resides in the erythrocytes and is thermolabile. Disruption of the cell by laking has no effect upon the inhibitor. 3. The bloods of animals closely related to the sheep, such as the goat and the cow, have a similar inhibitory action on the growth of hemolytic and non-hemolytic members of the influenza group, while human blood contains a similar but less powerful inhibitor for these organisms. 4. Members of the influenza group grow well on unheated rodent blood: rabbit, guinea pig, and rat. 5. These organisms also grow fairly well on unheated horse blood.