Depending on its concentration, phosphatidylglycerol, one of the three main Escherichia coli phospholipid species, is able to activate or inactivate the E. coli murein amidase (N-acetylmuramoyl-L-alanine amidase, EC 3.5.1.28) (Vanderwinkel, E. and De Vlieghere, M. (1985) Biochim. Biophys. Acta 838, 54-59). The mechanisms underlying the modulation of this enzyme activity were studied by analyzing the effects of cations, polycationic molecules, various surfactants and amphiphilic water-soluble compounds. K+, Mg2+ and polyamines were all able to prevent completely the enzyme inactivation produced by millimolar order concentration of phosphatidylglycerol. The efficiencies of the ionic species tested were in the order K+ less than Mg2+ = putrescine less than spermidine less than spermine. The kinetics of the counteraction processes were all sigmoidal. By contrast, the activation of the murein amidase produced by phosphatidylglycerol in micromolar concentration appeared to be insensitive to the ionic strength of the medium. Surfactants and amphiphilic molecules differing in their polar head and hydrophobic tail were found to activate the enzyme at various degrees for concentrations below their critical micellar concentration. The non-ionic surfactants were the most potent activators and remarkably mimicked the phosphatidylglycerol activation. The enzyme activation process appeared to require only a hydrophobic solvation shell around the protein. All kinetic data supported our previous interpretation of the phosphatidylglycerol-enzyme interactions in terms of multisite non-allosteric theory.