294 
Transactions of the Royal Society of South Africa. 
3. (a) B. diphtheriae, " strain 2 "+10 c.c. serum (from 2 
animals ; sera pooled) died 19th day. 
(b) control ......... 2nd „ 
4. (a) B. diphtheriae, " strain 3 "+5 c.c. serum (from 2 
animals), and 5 c.c. 
following day . 
. survived. 
(b) control 
. died 2nd day. 
5. (a) B. diphtheriae, 
" strain 3 " + 10 c.c. serum 
. survived. 
(b) control 
. died 2nd day. 
6. (a) B. diphtheriae, 
" strain 3 "+10 c.c. serum 
. „ 5th „ 
(6) control 
. „ 2nd „ 
7. (a) B. diphtheriae, 
" strain 3 " + 20 c.c. serum 
. „ 8th „ 
(b) control 
. „ 2nd „ 
8. (a) B. diphtheriae, 
" strain 3 " + 10 c.c. serum 
. survived. 
(b) control 
. died 3rd day. 
It is a phenomenon of considerable interest that the serum of one in- 
dividual of an animal species, which is highly susceptible to experimental 
B. diphtheriae infection, should be capable of affording some protection, 
when injected parenterally in certain amounts, to another individual of the 
same species infected with the particular organism. 
Experiments with other Proteins ; 
Bactericidal Experiments. 
Experiments were carried out with egg-albumen, milk, and peptone to 
determine whether the parenteral injection of these proteins would confer 
any protection as in the case of the serum proteins. The results were 
uniformly negative. 
In vitro bactericidal tests were also made to ascertain whether horse 
serum (in varying amounts), along with fresh guinea-pig's serum as comple- 
ment, exerted a specific bactericidal action on B. diphtheriae, but no such 
effect could be demonstrated, thus excluding the possibility of the pro- 
tection being due to a normal bactericidal immune body in horse serum. 
Experiments with Diphtheria Toxin. 
Experiments were then carried out to ascertain whether the protective 
effect was due to a natural antitoxic action on the part of the serum injected, 
though it seemed difficult to explain on this basis the action of guinea-pig's 
serum in the case of infection in the same species. 
A powerful toxin was prepared from B. diphtheriae, " strain 3." It was 
found that 10 c.c. of horse serum was protective against 10 M.L.D. of the 
