66 
The refined antitoxic serum, bulk for bulk, is quite as toxic to sensi- 
tized guinea pigs as the untreated serum from which it was precipi- 
tated and dial}^zed. There is, however, a distinct advantage gained in 
using the concentrated serum, as the same number of units may be 
given in half the bulk, and it is well known that the serum reaction in 
man depends partl}^ upon the quantity of serum given. 
Serum from one horse appears quite as toxic as serum from other 
horses. The apparent differences seem to depend upon something 
connected with the sensitizing action. 
The immunity produced by repeated injections, termed "‘anti-ana- 
phylaxis ” by Besredka and Steinhardt, appears to be relatively not 
quite as lasting and definite as man}^ instances of active immunity 
against bacterial infections. 
Hypersusceptibility to the toxic action of horse serum is transmitted 
through the female guinea pig. The male has no influence. 
The susceptibility is not transmitted through the milk. 
Maternal transmission of hypersusceptibility succeeds whether the 
female guinea pig is sensitized before or after conception. 
The phenomenon of hypersusceptibility appears to have no relation 
to aggressins. 
Methemaglobin is not present in the blood of guinea pigs dead of a 
second injection of horse serum. 
Ox}^gen has no influence upon the S}unptoms. 
Neither diphtheria toxine nor tetanus toxine appreciabl}^ influence 
the phenomenon of hypersusceptibilit}" produced by horse serum. 
The removal of the spleen or th}U’oid gland does not influence hj^per- 
susceptibility in the guinea pig. 
Guinea pigs fed upon beef are susceptible to a subsequent injection 
of cattle serum. 
Guinea pigs fed with cooked meat are not susceptible to subsequent 
injections of serum. 
When a second injection of horse serum is given directly into the 
heart of a susceptible guinea pig the S 3 miptoms are manifested with 
promptness and virulence. Under these circumstances, 0.01 c. c. 
(injected directly into the heart) in one instance was sufficient to cause 
the death of a sensitized guinea pig. 
Guinea pigs remain susceptible a very long time. There is no dimi- 
nution in the susceptibilit}^ of a guinea pig to subsequent injections of 
horse serum for at least one \^ear. The longest period we have 
observed is 480 days. 
We have never seen symptoms resulting from the first injection of 
horse serum in a guinea pig born of an untreated mother. 
The problem of hypersusceptibility has an important bearing upon 
the question of immunity. Our work indicates that hypersuscepti- 
bility is either an essential element or one stage in the process of 
