The ability of cecropin A to permeabilize and depolarize the membranes of Escherichia coli ML-35p bacteria has been compared to its bactericidal activity in an extension of earlier studies performed on synthetic lipid vesicle membranes (L. Silvestro, K. Gupta, J. H. Weiser, and P. H. Axelsen, Biochemistry 36:11452-11460, 1997). Our results indicate that differences in the concentration dependences of membrane permeabilization and depolarization seen in synthetic vesicles are not manifested in whole bacteria. The concentration dependences of both phenomena roughly correlate with bactericidal activity, suggesting that the bactericidal mechanism of cecropin A is related to membrane permeabilization.