Using the complement fixation reaction with whole bacterial solutions as antigens, it has been possible to confirm the existence, within the non-hemolytic streptococcus family, of two great groups.

One of these groups, that which may be designated as of the left side, is related in an orderly way to the hemolytic streptococci; the other of the right side, is similarly related to the entire pneumococcus family.

In addition, the hemolytic streptococcus family shows a reactive range which relates it, though less closely, to the non-hemolytic streptococci of the right side, and to the pneumococci.

The existence of these cross-relationships is not associated with the precipitable substance of either family, but may tentatively be presumed to be dependent upon the properties of the protein or lipoid-protein fraction of the bacterial cell.

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