Autocide AMV of Myxococcus xanthus was purified and identified as phosphatidylethanolamine. Alkaline hydrolysis of AMV yielded a high proportion of mono- and diunsaturated fatty acids. The bactericidal activity of AMV on M. xanthus depended upon the density of target cells: the greater the cell density, the greater the killing by AMV. For example, at 2 U of AMV per ml, 0, 50, and 99% killing was measured with 2 x 104, 2 x 105, and 2 x 107 target cells per ml, respectively. The cell-density-dependent activity of AMV was also observed on solid medium. Studies with model lipid compounds suggest that the inhibitory activity of AMV is due to the fatty acid moiety, released from phosphatidylethanolamine by the concerted (enzymatic) activity of many cells. Mutants of M. xanthus selected for resistance to AMI (a mixture of fatty acids) were also resistant to AMV. The possible role of AMV in developmental lysis is discussed.