Mice are inoculated with varying dilutions of serum 1 hr. preceding the inoculation of a young culture suspended in mucin. The culture is of maximum virulence. The dilutions of serum to be used are detd. by the intensity of plate precipitation reactions. The conc, of mucin is dependent on the variability of the test. The mice are from a pure inbred strain selected according to sex and wt. They are kept under observation for 96 hrs. A peritoneal exudate smear of each mouse that dies is examined. From the number of mice that survive and die. the dilution of serum which protects 509c of the mice is calculated by the Reed-Muench method. The value of the serum is expressed in relation to the control serum which is included in each test. This [image] test is recommended for the evaluation of antimeningo-coccus serum.