290 
Transactions of the Royal Society of South Africa. 
and intravenously in guinea-pigs simultaneously with diphtheria toxin 
always protects against 1 M.L.D., and may neutralise even 8 M.L.D. ; they 
showed that the activity of the serum depends on the proteins and not on 
the alcohol-soluble fraction. They regarded the protective power as not 
entirely due to a natural diphtheria antitoxin. 
In the writer's original experiments guinea-pigs of approximately 
350 grms. weight were injected subcutaneously with 2 c.c. of a 2-days 
bouillon culture of virulent strains of B. diphtheriae, and at the same time 
5-10 c.c. of normal horse serum were injected subcutaneously at a different 
site from that of inoculation ; these animals survived, while animals of 
similar weight, injected with the same amount of the same cultures, 
died within 24 to 48 hours, showing at autopsy the characteristic effects of 
B. diphtheriae inoculations. Various strains and different specimens of 
normal horse serum gave the same results. 
For the detailed investigation, particular strains of B. diphtheriae isolated 
from typical cases of throat diphtheria were used. 
Degree of Protective Action of Normal Horse Serum. 
In order to ascertain quantitatively the degree of protection conferred 
by normal serum, guinea-pigs of approximately equal weight were injected 
subcutaneously with a fixed quantity of a B. diphtheriae bouillon culture, 
and at the same time with varying amounts of horse serum ; the minimum 
lethal dose of the culture was also estimated in parallel experiment (Table I). 
Thus 2 c.c. of serum protected against at least 12 lethal doses of the 
culture (i.e. 1 c.c. serum against 6 M.L.D.). 
Table I. 
Guinea-pigs approximately 400 grms. weight injected subcutaneously in 
flank with 3 c.c. of a 2-days bouillon culture of B. diphtheriae, " strain 3 " ; 
varying amounts of normal horse serum injected subcutaneously in back. 
1. B. diphtheriae +6 c.c. serum . . survived. 
2. „ +5 c.c. „ . 
3. „ +4 c.c. „ 
4. „ +3 c.c. „'..'.. 
5. „ +2 c.c. 
6. „ +1 c.c. ,, . . died 6th day. 
7. „ +0-5 c.c. „ . 4th „ 
8. „ +0-25 c.c. ,, . „ 3rd „ 
9. „ +0-1 c.c. „ 4th „ 
10. ,, no serum . 1 „ 2nd ,, 
