1. For the particular toxin tested, the intravenous m.l.d. for mice was 60 times and the intramuscular m.l.d. for mice 100 times, the guinea-pig sub cutaneous m.l.d.

2. Antitoxin has the same neutralising power for toxin in mice as in guinea-pigs.

(1) An animal may be in a state of active immunity before any circulating antitoxin can be detected and, further

(2) The condition of active immunity continues after the disappearance of all circulating antitoxin produced by the animal in response to a previous stimulus.

The injection of the small amount of diphtheria toxin used in the Schick test may act as a secondary stimulus.

A Schick test may therefore cause a great and rapid increase in the immunity of the animals tested.

Examples are quoted of six rabbits and twelve guinea-pigs in Tables II to VII.

2. A fraction of a Schick dose may act as a secondary stimulus. Rabbit G 23 quoted in Table IV, injected with 1/10 of a Schick dose, showed an immunity response, the antitoxic content of its blood rising from 1/50 to nearly 1/10 unit per c.c. in six days.

3. The action of a Schick dose as a secondary stimulus may cause an animal to give a negative reaction when tested seven days or more after the first positive reaction.

This is illustrated by rabbits G 7 in Table III, G 31 in Table II, G 32 and G 34 in Table V and four guinea-pigs in Table VI.

4. The antigenic value of a Schick dose of toxin as a secondary stimulus may be as high as that of a reasonable dose of a toxin-antitoxin mixture suitable for human immunisation. Examples are given comparing the results of the injection of a Schick dose of toxin and of a toxin-antitoxin mixture in the same rabbit in Table III, in different rabbits, G 20 and G 22 in Table IV and reference is made to the companion rabbits to those quoted in Table V.

5. The antigenic value of a Schick dose as a secondary stimulus can be demonstrated:

A. In animals which have not produced a detectable quantity of antitoxin (that is less than 1/2000 of a unit per c.c.) as the result of a primary stimulus.

See both rabbits in Table II, rabbit G 20 in Table IV, both rabbits in Table V, and guinea-pig FF 19. v in Table VII. The four guinea-pigs in Table VI probably come under the same heading.

B. In animals whose actively produced antitoxin has fallen below a de tectable level.

See rabbit G 7 in Table III.

(These results add further confirmation to the phenomenon reported in the paper “Active immunity to diphtheria in the absence of detectable antitoxin” (Glenny and Allen, 1922).

6. A Schick dose of toxin which gives a positive reaction may, by acting as a secondary stimulus, produce a rapid increase in the antitoxic value of animals already containing some actively produced antitoxin.

See guinea-pig LL 17. vi in Table VII.

7. A Schick dose of toxin which causes no reaction may, by acting as a secondary stimulus, produce a rapid increase in the antitoxic value of animals already containing some actively produced antitoxin.

See guinea-pigs in Table VII.

8. A Schick dose of toxin may fail as a secondary stimulus if the antitoxic content at the time of injection is comparatively high.

See rabbit G 21 in Table IV.