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1. The influence of ionic strength, pH, basic amines and divalent cations on the apparent lytic activity of T4 lysozyme on chloroform-treated Escherichia coil B cells has been studied, and in some cases compared with the effect on hen egg-white lysozyme. 2. The lytic activity of T4 lysozyme was found to be markedly dependent on ionic strength and pH. Maximum activity was observed at pH 7.3 at I = 0.07. Very low activity was observed above pH 8.5 whereas maximum activity of hen egg-white lysozyme at I = 0.06 was obtained at pH 9.2. 3. Low concentrations of spermine, 40 μM, enhanced the lytic activity of T4 lysozyme 3 to 4-fold. No activating effect was observed on hen egg-white lysozyme. 4. Higher concentrations of spermine, above 100 μM inhibited lysis, presumably by stabilizing the protoplasts formed. The muramidase activity of T4 Iysozyme was not inhibited, but rather stimulated also under these conditions. 5. Several other amines and Mg2+ and Ca2+ were also tested. They exhibited similar effects as spermine. In contrast to spermine, the inhibition observed at high concentrations of Mg2+ is presumed to be due to inhibition of the enzyme. [ABSTRACT FROM AUTHOR]