In the titration of an antibiotic substance it has been usual to choose for the test organism a microbe which. is very sensitive and which is easy to work with. For streptomycin use has been made of Gram-negative bacilli such as B. coli or Friedlander's bacillus, and one strain of Friedlander's bacillus (Klebsiella 41) has been extensively employed in Arnerica. In this article we propose to deal with some of the difficulties we have encountered in the titration of streptomycin in solution or in the patient's serum. We might say here that, as we had no official standard solution of streptomycin, we used for these tests a standard solution made up by dissolving in 100 ml. the contents of one bottle of Merck's streptomycin, which was labelled as containing 1 g. of streptomycin hydrochloride.Influence of the Nature of the Culture Medium For some time we have been using glucose-phenol-redserum water for the titration of penicillin in patients' serum. When streptomycin was titrated in this medium we found that we obtained a much higher end-point than when it was titrated in ordinary broth. We therefore made observations on the bacteriostatic power of streptomycin on a number of different organisms in four different medianamely, (a) ordinary digest broth; (b) peptone water; (c) glucose-phenol-red-serum water (this is made by boiling for a few minutes serum, 2 parts; distilled water, 6 parts; 10% glucose solution, 2 parts; and saturated solution of phenol red, sufficient to give a red colour); (d) defibrinated human blood, inactivated.In regard to the first three media serial twofold dilutions of the stock streptomycin were made in the media which had been inoculated with 10 c.mm. per ml. of a 24-hour culture of the test organism. This was done in 0.5 ml. quantities in test-tubes. With broth and peptone water the end-point was the appearance of-visible growth after 24 hours. In the serum water there was a colour change from red to yellow, and a precipitation of the medium where the organisms grew and fermented the glucose.When the test was made in blood the streptomycin was diluted in saline in 25-c.mm. volumes; these were mixed with the same volume of blood which had been suitably infected with the test organism, and incubated in slide cells. As the growth of Friedlander's bacillus in blood is not very obvious 2% glucose was added when testing this organism: When growth then occurred the blood was haemolysed and a good end-point obtained. The results are shown in Table I.

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