A single serological type of non-hemolytic streptococcus, designated streptococcus MG, has been isolated from the lungs of fatal cases of primary atypical pneumonia, from the sputa of patients with this disease, and occasionally from the respiratory tracts of normal human beings. Certain biological characteristics of this microorganism have been studied. All of the 59 strains isolated have been shown to belong to a homogeneous bacteriological group with characteristics which serve to distinguish it from any other well defined species of streptococcus. It has been suggested that this microorganism probably represents a distinct and hitherto undifferentiated species of streptococcus.

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