The experiments reported here show that the infection-enhancing factor of testicle extract is neutralized in vitro by an antiserum against homologous testicle extract. An antiserum developed against a testicle extract of one species does not influence the enhancing and spreading factor of the extract from another species. Rabbits immunized against testicle extract do not exhibit any alteration in the spreading or enhancing effect of extracts employed later, even when the testicle extract used is from the same species as that employed for the immunization. Whether the in vitro inactivation of the active factor is a specific neutralization, or is the result of adsorption of the factor on the flocculate formed, has not been definitely determined. The fact that there is no neutralization in vivo, and that the antiserum acts only on extracts from the same species, when definite flocculation takes place, tends to emphasize the probability that the neutralization is not a direct one, but is incidental to the flocculation mentioned.
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