A small percentage of adult rabbits is refractory towards the elicitation of the Shwartzman phenomenon. There is a definite relationship between the type and amount of toxin employed and the percentage of positive reactions. Shwartzman (1) reported that about 30 per cent of rabbits are refractory if typhoid agar-washing filtrate is used and only a few per cent with meningococcus agar-washing filtrate. It seemed of interest to determine whether constitutional factors, such as age, may play a rôle in the Shwartzman phenomenon. One litter of a few days old rabbits did not display the Shwartzman phenomenon in an experiment carried out by Gratia and Linz (2). Freund and Hosmer (3) observed that the appearance of the hemorrhagic local skin reaction depends upon the age and weight of the rabbits tested. A positive reaction was obtained in young rabbits weighing between 1100 and 2000 grams only after the reinjection of the bacterial agar-washing filtrate, but not after the reinjection of starch.