The potential importance of streptomycin (Schatz et at., 1944) as a therapeutic agent has made the study of this antibiotic an urgent problem. In order to expedite progress the Abbott, Eli Lilly, Parke Davis, and Upjohn research laboratories have cooperated in a joint attack on the problem, and the Departments of Chemistry and Horticulture of the University of Illinois have contributed to certain phases of the work. One of the first problems to be investigated was that of developing an assay method which would be satisfactory, not only for beers, but also for solutions of streptomycin containing organic solvents and reagents. Waksman and coworkers have used the agar-dilution method in defining the currently accepted Escherichia coli streptomycin unit (Schatz et al., 1944). The agar-diffusion method has been recommended for an assay of potency (Waksman, 1945) and has been applied to the determination of streptomycin in body fluids (Stebbins and Robinson, 1945). A slide-cell technique for the assay of body fluids also has been described (Heilman, 1945). Since considerable difficulty was encountered in obtaining consistent results with the agar-diffusion method, it seemed desirable to make our experiences available to other workers in the field. In this paper we are presenting studies on the important sources of variation and the procedure currently employed in our laboratories for the assay of streptomycin. PROCEDURE Medium. Bacto-streptomycin assay agar, dehydrated, was prepared by the Difco Laboratories, Inc., in collaboration with our group, to provide a supply of a standard, uniform medium for the assay of streptomycin.1 The medium is prepared for use by dissolving 25.5 g per 1,000 ml in doubly distilled water. After sterilization the medium may be stored at 2 to 4 C until used. 1 The authors gratefully acknowledge the cooperation of Mr. H. G. Dunham of the Difco Laboratories, Inc., in making this medium available.

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and St. John (2), after a very long peried of digestion, were 97.1 and 96.9% for methionine and cystine, respectively.

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