Addition of 100 micrograms/ml of colistin (polymyxin E) to an actively-growing culture of Mycobacterium aurum caused apparent loss of viability starting after 20 min of drug addition, whereas significant Mg++ efflux was apparent within 5 min of drug addition. Therefore, Mg++ efflux preceded cell death by at least 15 min. These data confirmed earlier reports showing that colistin interacted with the cytoplasmic membrane of mycobacteria.