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SUMMARY: Bacitracin at concentrations of 6·5–13·0 µg./ml. inhibited the growth of Mycobacterium smegmatis atcc 607. Concentrations 10 times higher were required to produce a similar effect on M. tuberculosis bcg. Assays of the bacitracin effect in Sauton medium showed no significant drug inhibition due to the cancelling effect of citrate ions present in this medium. Low concentrations of bacitracin produced smooth-surface colonies consisting of bacteria which showed a modified cell-wall appearance, in contrast to the rough colonies formed by control bacteria. Ultrastructure studies indicated that the main target of bacitracin action on mycobacteria may be a direct effect on the membrane system.