194 MORTEN SIMONSEN 



consists of 4 immune donors. Two of the donors have been 

 tested in 2 htters, but in these cases the mean values of the 2 htters 

 are used, so that each immunized donor (and its normal control 

 donor) is represented by one set of data only. 



This leaves us with a block of spleen indices analogous to Table 

 I except that it consists of only 4 horizontal columns. These 

 columns are now summed vertically, as was done with all the 

 data of Table I. The F.I. is then calculated on the basis of the 

 mean values, whereby sampling errors of individual test litters 

 largely cancel each other out. The group of repeatedly injected 

 donors is treated in the same manner. The results are entered in 

 Table II together with similar results obtained from the 3 other 

 strain combinations which have been studied. 



The C3HX AKR F^ hybrids have presented special difficulties 

 which stem from the fact that normal C3H cells, even in very 

 high doses, only produce a small degree of spleen enlargement 

 (almost invariably an index below i* 50). The actual procedure 

 adopted for this combination (in contrast to the three others which 

 followed the pattern described for the combination with ST/ A) 

 was to make a 3 -point titration with preimmunized C3H cells 

 (doses: 0-5 ,1-5 and 4- 5 milhon), and a i-point determination of 

 the response to normal C3H cells (20 milhon). The immune cell 

 doses were of course supplemented to a total of 20 million by the 

 addition of inert cells. The F.I. was eventually determined as the 

 horizontal distance between the immune line and a line drawn 

 parallel to it, through the mean response to 20 million normal 

 cells. The assumption of parallelhsm in this combination rests 

 therefore entirely on analogy with the 3 other combinations, 

 where the difference in slopes between normal and immune curves 

 did not deviate significantly from zero. 



Comparison of F.I. in 4 strain combinations 



Table II, column 5, shows clearly that the F.I. differs in C3H 

 mice in response to the 4 foreign strains which have been tested. 



