l88 MORTEN SIMONSEN 



The two cell suspensions are adjusted, on the basis of preHminary 

 experiments, so that their relative density can be expected roughly 

 to compensate for their difference in potency. For example, if 

 immune cells can be expected to be about 2-5 times as effective 

 as normal cells, the latter suspension is always prepared three 

 times as concentrated. 



Most assays in the present investigation have been 6-point 

 assays, i.e. the two suspensions have been tested in three dosages 

 each. As the two suspensions are naturally best compared in one 

 and the same litter, it means that only litters of seven or more 

 members are suitable test objects, number 7 (and possibly also 8) 

 serving as controls injected with isogenic spleen. A litter size over 

 8 is always reduced to that number, as bigger litters often tend to 

 be malnourished. Obvious congenital runts are discarded before 

 injection. 



The doses of each suspension are always spaced logarithmically 

 to facilitate later computation (cf. Table I). 



If, as in Table I, the highest dose injected into any litter 

 member is 9 million cells, all other doses are made up to 9 million 

 by the addition of the required number of inert cells; for which 

 purpose spleen cells from an adult Fi hybrid, isogenic with the 

 test litter, are used. The reason for doing this is that (unpublished) 

 experiments, together with Dr. D. Michie, have indicated that a 

 mechanical saturation effect may be operating if the titration is 

 not done in a constant total cell number. It is easy to visuahze 

 that an infantile mouse spleen will accommodate a decreasing 

 fraction of the injected cells as the dose is increased. By observing 

 the condition of a constant total cell number it is supposed that 

 the same fraction of the inoculum settles in the spleen in all 

 animals, so that the ratio between dose levels as prepared in vitro 

 will actually be maintained in vivo. 



Test htters are killed 8-10 days after the injection, and spleen 

 indices determined as described earlier (Simonsen 1962, and 

 previous publications referred to there). 



