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HE potencies of antibiotic substances have been measured by several Td ifferent methods. The most simple and perhaps the most widely used technique is that involving the serial dilution of the sample with agar1 or with broth2, a and the noting of the last dilution a t which complete inhibition of the test organism occurs. Turbidimetric assays4? are merely refinements of the dilution assays wherein the antibiotic substance is diluted in such a manner as to produce an end-point which will fall intermediate between complete inhibition and no inhibition. Agar diffusion methodse similarly involve applying appropriate dilutions of the antibiotic in question to an inoculated agar surface and, after suitable incubation periods, measuring the zones of inhibition. With any assay method, it is important that a control solution be assayed in parallel with the "unknown" samples in order to have a point for comparison.When the above methods were used in an attempt to assay aureomycin solutions, it was observed that the agar diffusion techniques as we applied them failed to produce sharp zones of inhibition. Also, the slope of the inhibition response curve was so low as to indicate a rather low order of precision. In making a survey of the various organisms readily available to us in the laboratory, we noted differences in the sensitivity of these organisms to aureomycin when tested by dilution methods. Since in selecting a test organism we must consider factors other than sensitivity alone, we eliminated those organisms which might be regarded as pathogenic. We did this not only for reasons of safety of laboratory personnel, but since we were primarily interested in assaying blood specimens we wished to eliminate the possibility of interference from specific bacteriostatic substances present in blood. In addition to the authentically identified cultures from our collection, we tested a number of laboratory isolates. One of these, Bacillus No. 5, seemed to meet our specification. It is a gram-positive spore-forming bacillus and appears to be a strain of Bacillus cereus variety mycoides. ' The method finally adopted for the assay of serum samples is similar to the methods originally proposed by the Food and Drug Administration for the assay of penicillin* and streptomycin9 in blood. One-half ml. of a 0.2 pg./ml. solution of an aureomycin reference solution is placed in a ml. portion of the same solution is diluted serially two-fold through an additional 5 tubes with sterile nutrient broth.1° Samples submitted for assay are similarly diluted [ 218 1x 434 inch Wassermann tube. A second

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