1. 14 day old chick embryos are protected against subsequent meningococcus infection through the amniotic route by the intravenous administration of an homologous antiserum produced in hens, by a commercial concentrated polyvalent meningococcus antiserum and by a commercial meningococcus antitoxin.
2. Titrations of the different antisera indicate that the homologous and commercial polyvalent sera have approximately the same protective value and are much more effective than the commercial antitoxin. The titrations also show that the chick embryo is a sensitive indicator of the amount of antibodies present in a given amount of serum.
3. The mechanism of the protective action of the antisera is not apparent from these experiments except that in the treated embryos there is a relative inhibition of the growth and presumably a neutralization of the injurious products of the meningococci.
