THE FACTOR OF IMMUNIZATION I97 



the cardinal signs of overt runt disease, weight retardation and 

 death, would establish the same ranking of antigenic strength. 



For this purpose, members of o-i-day-old F^ hybrid litters 

 were each injected i.P. with 25 million adult C3H spleen cells. 

 One, or usually, two litter-mates were kept as non-injected 

 controls. The period of observation was 30 days, as death would 

 have usually occurred by then if at all. Mice which died before 

 day 5 after injection were discarded as having probably died from 

 reasons other than runting. All litters were inspected daily for 

 recording of deaths, and all survivors at day 21 were weighed on 

 that day. 



The dose of 25 million C3H cells was chosen because earlier 

 experiments (with a different purpose) had already suggested that 

 this dose produced a higher mortality in C3H hybrids with DBA/2 

 than in similar hybrids with ST/ A. 



It has been a great weakness in the utilization which has been 

 made so far of data for death and weight retardation in runt 

 disease, that no attempt has been made to combine the two kinds 

 of information in a single formula. 



When a less than 100 per cent lethal dose is injected, the charac- 

 teristic outcome will necessarily be that some litter-mates show 

 more or less weight retardation, while others die at different 

 times. How is the combined result from such a htter to be 

 expressed numerically ? 



Table III represents a crude attempt at solving this problem, and 

 gives, at the same time, the data for that litter in the series with the 

 severest runting compatible with a life-span corresponding to the 

 period of observation. This animal had, at day 21, a body weight 

 which was 40 per cent of its norinal control, or a ratio of o- 40. 



We shall now assume that it was a matter of sheer luck that this 

 mouse was still alive at day 30. If we equilibrate its degree of 

 runting to that of a hypothetical litter-mate which would have 

 died at day 30, both mice would be given an index of runting of 

 0-40. 



