1. The bactericidal activity in vitro of different antimeningitis sera was found to be quite low. Fresh or active antimeningitis sera was somewhat more bactericidal than the same sera after inactivation by heating at 60°C. for thirty minutes.

  2. Active normal human and guinea-pig sera are generally slightly bactericidal for meningococci.

  3. The bactericidal activity of horse-antimeningitis and normal human and guinea-pig serum, is largely independent of complemental bacteriolysis.

  4. The bactericidal activity in vitro of normal and immune sera was best shown by a pipet method employing small numbers of cocci and relatively large volumes of serum.

  5. The addition of active normal human and guinea-pig serum to antimeningitis serum sometimes increased the bactericidal activity of the latter.

  6. Whole human and guinea-pig blood was found slightly more bactericidal than the sera alone.

  7. Normal human and guinea-pig sera frequently agglutinates meningococci in final dilutions up to 1:4, but not in higher dilutions.

  8. Antimeningitis sera containing the largest amounts of agglutinin were found to possess most opsonin and apt to prove most bactericidal in vitro.

  9. The mouse test of Hitchens and Robinson was found under certain technical conditions to be of value as a means for determining and measuring approximately the protective and curative value of antimeningitis serum.

  10. Normal active human and guinea-pig sera were practically without demonstrable protective value in mice infected with virulent meningococci, although the addition of these normal sera to antimeningitis serum appeared in some experiments to slightly increase the protective power of the latter.

  11. The addition of active normal human or guinea-pig serum (complement) to antimeningitis serum cannot be expected on the basis of our experiments, to greatly augment bactericidal activity because complemental bacteriolysis exerts but a minor rôle in the relatively feeble bactericidal activity of antimeningitis serum, but the addition of normal sera definitely increases opsonic activity (10) and since it would appear that a large part of the curative properties of antimeningitis serum is to be ascribed to the presence of opsonin, it is suggested as worthy of clinical trial to complement the antimeningitis serum by the addition of active human or guinea-pig serum prior to intraspinal injection and particularly in the treatment of severe and serum-resistant infections.

For this purpose human serum is superior to guinea-pig serum and may be obtained from the patient or a volunteer with the usual precautions for asepsis.

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