It has been reported that the inhibitory action of streptomycin on bacteria can be counteracted by culture medium components, pH, salts, etc. (1-4). In many instances, however, these results were obtained by seeding a small inoculum of organisms into media containing varying concentrations of the antibiotic and the material tested, and determining the occurrence of growth after a period of incubation of approximately 24 hours. Little cognizance was taken of the metabolic activity of the organisms surviving the antibiotic treatment, although this may differ markedly from that of the parent organisms. Growth may have resulted from (1) selection of a few resistant organisms, (2) adaptation of the majority of cells in the original inoculum to alternate metabolic pathways, or (3) a general stimulatory effect of the added test compound which would increase growth of normal cells as well as that of cells exposed to streptomycin.To demonstrate the true reversal of the antibiotic action of streptomycin it should be shown that (I) almost all the org~tni.~ms present (at least 99.9 per cent) are injured after a short exposure to the antibiotic, (2) in the presence of the reversing agent, the viable count of the streptomycin-treated culture returns to approximately that of the control, and (3) the duration of exposure to the reversing agent is short enough to preclude the possibility of selection of resistant organisms or the adaptation of the culture to the new conditions.To fulfill these requirements it would appear desirable to utilize a culture in the logarithmic growth phase, carried in a chemically defined medium. The presence of an inhibitor, therefore, would interfere with the normal activities of actively metabolizing cells, and the organisms would be injured in such a manner that death would follow quickly unless some interruption of the process were introduced. Reversal of this pathological condition could be accomplished by the removal of the toxic agent and the addition of substances which could be