1. It has been found by the pipet method that normal rabbit blood and serum are capable of killing considerable numbers of virulent normal meningococci in vitro within three hours.

  2. The meningococcidal activity in vitro of normal rabbit blood was found to be increased up to a certain limit by the intravenous injection of the living and autolyzed meningococci. After that, further immunization did not appear to increase bactericidal activity, was generally rather irregular and not infrequently even decreased meningococcidal activity of the blood. The more highly immunized rabbit's blood was found sometimes less bactericidal than that of slightly immunized rabbit's blood.

  3. The meningococcidal activity of normal rabbit's serum has been found not to be increased by artificial immunization and also to be comparable with that of defibrinated blood of an immune rabbit.

  4. The meningococcidal activity in vitro of immune rabbit's blood was found by the pipet method to be distinctly stronger than that of the serum, of defibrinated blood or of blood consisting of blood cells and serum or of citrated blood. It was suspected that at least one factor in explaining the higher meningococcidal activity in vitro of immune rabbit's blood compared with defibrinated blood, citrated blood and serum lies in the influence of coagulation of the blood, which is permitted in the regular blood test as described, favoring the phagocytosis of meningococci.

  5. The meningococcidal blood test can not be accepted on the basis of the present investigation for the purpose of measuring or determining the artificially induced immunity against meningococci.

However, as pointed out by Dr. Heist (1) the bactericidal blood test described possesses the advantage of employing whole blood; hence when this method is used any mechanism of immunity existing in the blood, may be brought into direct relation with the microorganism. Moreover, in this test any antibacterial factors existing in the blood, and the influence of individual variation of fluid constitutent as well as phagocytes in blood, upon the bactericidal activity of the blood, have equal chance to come into play. Furthermore, it was found that a parallelism between the bactericidal activity of the blood and resistance to certain bacteria including meningococci, exists under normal conditions (1) (2). Therefore, the bactericidal blood test described may be regarded as a method possessing definite value for the measuring the natural resistance of the organism for certain microorganisms especially for meningococci, inasmuch as with respect to meningococci no accurate method sufficiently reliable to serve as a definite measure of antibody content has yet been devised for measuring immunity to this microorganism (16).

I wish to express my thanks to Dr. Yutaka Nakamura for advice and asistance in carrying out this work.

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