1. Four hundred roentgens of x-irradiation to an entire hind limb of young rats is sufficient to retard growth of the bones directly irradiated.
2. Eight hundred roentgens to an entire hind limb of rats retards bone growth in the area of direct irradiation and, abscopally, retards skeletal and somatic growth.
3. Eight hundred roentgens to the knee area of one extremity retards the growth of bone directly irradiated, but does not produce a demonstrable abscopal effect on skeletal growth.
4. Seventy-two days after irradiation, the weights of the rats that received 800 r to an entire hind extremity revealed a mean decrease of thirty-four grams compared with the mean weight of the control rats. The animals that received 800 r to the knee area or 400 r to the hind extremity exhibited a weight decrease of approximately twenty grams compared with the mean weight of the controls.
5. The severity of the abscopal effect is related to the dosage of irradiation administered and to the volume of tissue exposed to the irradiation.