These assay results are in line with the observations of others (3b•0•12) on the instability of B12b in the presence of certain reducing agents and assay media constituents such as thiamin and ascorbic acid. The apparent differences in microbiological assay attributed to vitamins Bi2a and B12b7 may well be the result of assay methods.Acknowledgments.-The production and isolation of vitamin Bi2b is obviously the result of group effort by many individuals. We are grateful to Dr. G. F. Cartland for active direction of the program, to Dr.

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Communications to the Editor 2947 140°, [ ]2 -51.4, was identical with the antibiotic obtained from the Streptomyces fermentation.

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