By a series of quantitative assays of concentrates prepared in a variety of ways, it has been shown that the previously described growth factor now called "strepogenin," required by certain hemolytic streptococci, is very similar to and possibly identical with the recently described growth factors for Lactobacillus casei and Smith's Streptococcus lactis. An improved procedure for assay of strepogenin with the aid of Lactobacillus casei has been described, and additional properties of the growth factor have been presented.

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