The technique of lyophilization has been used to prepare β-lysin that can be stored in aliquots at ordinary refrigeration temperatures. It has permitted the determinations of environmental ionic strength, pH, and magnesium ion concentration favoring bactericidal and lytic activities of this agent. Killing of cells as well as lysis of protoplasts ofBacillus subtiliswere favored in environments not more than 0.2 nor less than 0.15 in ionic strength. Of the environmental pH tested (6.0, 7.0, and 7.5), lysis of cells and of protoplasts was most efficient at pH 7.5, whereas bactericidal action was relatively unaffected in this range. The effectiveness of bactericidal action and of lysis of protoplasts decreased as the amount of Mg2+in the environment increased.