258 HILDEMANN, LINSCOTT AND MORLINO 



at 0-4 days of age with A/Jax lymphocytes, showed no evidence 

 whatever of graft-versus-host reactions was unexpected. The 

 apparent hybrid vigour of the hosts and failure of the injected 

 cells to multiply and thus intensify the attack may account for this 

 result. Experiments employing higher doses of donor lympho- 

 cytes are now in progress, hi this connexion, Trentin's (1958) 

 observation that lymphoid tissue from C57 parents had only a 

 shght effect on 21-day-old (C57 x A) Fi hybrids, whereas a 

 comparable dose of A-strain cells induced profound disease, 

 suggests that the particular strain combination involved may be 

 important. Contrary to simple immunogenetic expectations, the 

 age of Fi recipients receiving parent strain lymphoid cells has 

 turned out to be an important variable, hi our experiments, 

 severe runt disease was induced by small lymphocytes only in 

 recipients less than 48 hours old at the time of injection. With 

 another strain combination, Simonsen and Jensen (1959) found that 

 liver and spleen enlargement regularly occurred only in those Fi 

 animals which were less than 11 days old at the time of injection 

 with 10^ parent strain spleen cells. Similarly, Kaplan and Rosston 

 (1959) determined that very young (one to ten day-old) Fj hybrid 

 recipients were more sensitive than adults, but that disease occurred 

 in adults after injection of massive doses of parental strain lym- 

 phoid cells (see also Cole and Elhs, 1958). However, Trentin 

 (1958) found that parental strain bone marrow cells in high dosage 

 failed to cause weight loss or death in young adult Fj hybrids. 

 Considering the available evidence as a whole, it appears that 

 relative to host age or degree of development, much higher doses 

 of parental lymphoid cells may be required to cause deleterious 

 changes in young adult or adult F^ hybrids. Hybrid vigour is 

 perhaps responsible for the increased resistance of older animals. 

 Further investigation should reveal the basis for these anomalous 

 results. 



The donor cell preparations employed in these studies contained 

 no leucocytes other than blood lymphocytes. These lymphocytes 



