256 HILDEMANN, LINSCOTT AND MORLINO 



(1958) found mean survival times of 16 and 21 days, respectively. 

 Although Billingham and co-workers (i960) give no median or 

 mean survival times for neonatal rats inoculated with various types 

 of homologous, adult lymphoid cells, the range of ages at death 

 suggests that the time course of runt disease in this species is quite 

 similar to that observed in mice. Our early dosage experiments 

 suggested that A/Jax recipients less than eight hours old were 

 more susceptible to severe immunological attack than those 

 >8 <24 hours old. This supposition was not upheld by later 

 work. Apparent variations in susceptibihty of whole htters 

 probably reflect differences in the length of the gestation period 

 and consequent extent of immunological maturation at birth. 



The weight-gain and organ-enlargement assays each have 

 distinct advantages. The former permits long-term observation, 

 compilation of survival time data, and tests for tolerance and 

 chimerism ; the latter facilitates determination of detailed patho- 

 logical changes as a function of age and time after injection, 

 although a large number of experimental animals may be 

 required to permit sampling at various ages. 



Although high mean hver indices and low mean body weight 

 indices most sensitively reflected the severity of runt disease in 

 our system, Simonsen and co-workers (1958) generally found 

 spleen indices higher than liver indices and only slightly depressed 

 body weight indices in a variety of strain combinations. However, 

 their data are based on Fi hybrid recipients inoculated with 

 parental spleen cells at ages ranging from 0-14 days. Moreover, 

 they noted that liver enlargement was the more pronounced the 

 younger the host. Other pathological features observed in our 

 experiments are for the most part consistent with the fmdings of 

 others (Billingham and Brent, 1959; Kaplan and Rosston, 1959; 

 Siskind, Leonard and Thomas, i960; Russell, i960; Oliner, 

 Schwartz and Dameshek, 1961) who employed mixed populations 

 of donor leucocytes. The more profound changes observed by 

 Gorer and Boyse (1959) in the spleen and hver of Fj (C57BL x A) 



