1. The course of elimination of passive immunity in rabbits injected intravenously with diphtheria antitoxin obtained from a horse, consists of three phases:

(a) an initial loss of 50 per cent, occurring within the first 24 hours;

(6) a gradual constant percentage loss of approximately 25 per cent, from day to day, lasting 6–7 days;

(c) a rapidly accelerated loss of 50 per cent, or more per day after the seventh or eighth day.

2. In rabbits sensitised with small doses of horse serum the same three phases are seen, but

(a) while the initial loss remains the same

(b) the gradual constant percentage loss is of the same magnitude, but lasts only two or three days;

(c) the rapidly accelerated loss occurs after the third or fourth day and over 90 per cent, of the antitoxin present is lost within 24 hours.

3. Possible explanations of the three phases are:

(a) initial loss due either to precipitin to horse serum normally present in rabbits or to saturation of or adsorption to the tissues;

(b) a phase independent of precipitin action;

(c) accelerated loss due to formation of precipitin commencing seven or eight days after injection of horse serum in normal rabbits and three or four days after injection in sensitised rabbits.