262 HILDEMANN, LINSCOTT AND MORLINO 



Both weight-gain and organ-enlargement assays were 

 employed for quantitative evaluations of small-lymphocyte- 

 induced disease. Curtailment of mean weight increases as well 

 as low mean body weight and high liver indices sensitively 

 reflected pathological changes associated with graft-versus-host 

 reactions. The organ enlargement is attributed to host cell 

 multiphcation and reaction to immunological attack. 



Radioautography of lymphocyte preparations following expo- 

 sure to tritiated thymidine showed no labelling whatsoever. 

 A/Jax survivors of runt disease were not chimeras, but possessed 

 only A/Jax cells as evidenced by cytotoxic isoantibody tests. 

 Moreover, animals that recovered from runt disease were not 

 tolerant of C57BL/6 skin homografts. Conversely, A/Jax new- 

 borns inoculated with foetal C57BL/6 hver cells showed no 

 runt disease, but became highly tolerant of adult C57BL/6 skin 

 homografts. It is concluded that small lymphocytes from adult 

 mouse blood are immunologically reactive, though non-pro- 

 liferating cells. 



Acknowledgements 



We are most grateful to Dr. Charles Craddock for advice and assistance, 

 especially with the tritiated thymidine-radioautography experiments. We are 

 also indebted to Dr. Roy Walford for valuable technical suggestions. 



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