196 MORTEN SIMONSEN 



The four strains can be ranked in order of DBA/2, ST/ A, A and 

 AKR, beginning with the lowest values. 



The differences are particularly marked when hyperimmunized 

 mice are compared. Regarding the F.I. after a single immunizing 

 injection, there is still a significant difference between DBA/2 and 

 ST/ A on the one hand, and A and AKR on the other hand. 



Column 6 of Table II indicates the numbers of normal C3H 

 spleen cells which, on the average, are necessary to produce a 

 spleen index of 2- 00. These figures have been estimated by inter- 

 polation from the straight lines determined by the respective 

 *'semi-platoiiic" slopes and the points of mean dose-response for 

 all titrations of normal cells (the N-curve of Fig. i for the injection 

 of C3H cells into C3HX ST/A Fj hybrids). 



By dividing the figures of column 6 by their corresponding 

 F.I. as determined in hyperimmunized donors (column 5), the 

 figures of column 7 are obtained, indicating how many hyper- 

 immune cells are, on the average, needed to produce the same 

 spleen enlargement. It is quite apparent that the great bulk of 

 variation in the F.I., if not the whole, resides in the difference with 

 which normal C3H cells react upon different antigens. It is re- 

 markable how similar the figures are, in column 7, for DBA/ 2 and 

 ST/ A, and likewise for A and AKR. Exactly the same statement 

 applies to column 8 which gives the "semi-platonic" slopes. 



Antigenic strength in terms of runting power 



It is well established that splenomegaly is a characteristic sign in 

 the early phase of runt disease. It follows, therefore, that column 

 6 of Table II, which gives the estimated numbers of normal 

 C3H spleen cells necessary to produce a doubling of the spleen 

 weight in the 4 different F^ hybrids with C3H, gives an indica- 

 tion of the relative strength which the 4 different strain antigens 

 represent vis a vis C3H. It appears that DBA/2 represents the 

 strongest antigen, followed by ST/ A, A and AKR, in this order. 



It was, nevertheless, thought desirable to investigate whether 



