Since the discovery of Miller1 that suspensions of mucin greatly enhance the virulence of meningococci for mice, the mouse protection test has been used by a number of investigators2 as a method for titrating the protective value of antimeningococcus serums. There is general agreement that this constitutes the most satisfactory method of standardization. However, it is obvious, as will be shown by the details of the test, that such a method, because of the time and expense involved, is not applicable to the routine testing of strains prior to treatment with the serum selected. The following questions must therefore be answered before the test can be accepted as a reliable method for determining the therapeutic efficiency of the serums available at the beginning of an epidemic. If a serum produces good protection in mice against one type I strain, can one expect good protection in mice against all type I