The incidence of natural diphtheria antitoxin in horses: its influence on the results of antigenic stimulus

Out of 221 horses tested 34·8 % showed natural diphtheria antitoxin to the extent of 0·02 unit or more per c.c. of serum. 2. Amongst the horses in which no natural circulating antitoxin was detected, a much larger number failed to respond to stimulus than amongst those having circulating antitoxin. But, amongst the animals that did show appreciable response to primary stimulus, there was not much difference in degree of response between the horses with and those without natural circulating antitoxin.