Observations on the Protective Action of Normal Serum. 295 
toxin (1 c.c. against 1 M.L.D.), whereas 1-5 c.c. of diphtheria antitoxin, 
representing 1000 immunity units, protected against 10,000 M.L.D. The 
minimum lethal dose was estimated in parallel experiments along with 
the protective tests (Table VII). 
A similar series of experiments were carried out in which the pooled 
serum of three guinea-pigs was substituted for horse serum (Table VIII). 
The parenteral injection of 10 c.c. of guinea-pig's serum had apparently 
no protective action in guinea-pigs against B. diphtheria toxin. 
Table VII. 
Guinea-pigs 250 grms. weight injected subcutaneously in flank with 
"varying doses of diphtheria toxin ; at the same time injected subcutaneously 
in back with 10 c.c. normal horse serum. 
1. 
Toxin 0-1 c.c. + 10 c.c. serum . 
died 2nd day. 
2. 
„ 0-05 c.c.+ 
„ 2nd „ 
3. 
„ 0-01 c.c.+ 
„ 7th „ 
4. 
„ 0-005 c.c.+ 
„ 12th „ 
5. 
„ 0-001 C.C.+ 
survived. 
6. 
„ 0-0005 c.c.+ 
7. 
„ 1 c.c. + 1000 immunity units 
diphtheria antitoxin 
M.L.D. Estimation. 
1. 
Toxin 0-1 c.c. . 
died 2nd day. 
2. 
„ 0-01 c.c. . 
„ 2nd „ 
3. 
„ 0-001 c.c. . 
,', 2nd „ 
4. 
„ 0-0005 c.c. . 
„ 2nd „ 
5. 
„ 0-0001 c.c. . 
„ 3rd „ 
6. 
„ 0-00005 c.c. . 
survived. 
7. 
„ 0-00001 c.c. . 
M.L.D.— 00001 c.c. 
Note. — The small doses of toxin 0-05 c.c.-0-00001 c.c. were measured 
by preparing decimal dilutions of the toxin 1 : 10, 1 : 100, 1 : 1000, 1 : 10,000, 
1 : 100,000 and injecting the appropriate quantities of these dilutions. 
Table VIII. 
Guinea-pigs 250 grms. weight injected subcutaneously in flank with 
varying doses of diphtheria toxin ; at the same time 10 c.c. of guinea-pig's 
serum (from three animals, pooled) injected subcutaneously in back. 
