142 



HOWARD, MICHIE AND WOODRUFF 



Spleen assay method of Simonsen and Jensen (1959) was used, with 

 some refinements introduced to facilitate quantitative estimation. 

 The full design is shown diagrammaticallyin Fig. i and the results 

 are summarized in Table II. The two assays for which rough 

 figures only are printed are those in which imperfections of design 

 or mishaps of execution prevented precise estimation. In order to 

 illustrate the principles used, the measurements obtained from the 

 remaining two assays have been plotted in Fig. 2. Since 



an 



Table II 



Estimates of the host component of the immunologically competent cell 

 population present in the spleens of four of the graft-bearing a-strain mice 



(Data from Michie and Woodruff, 1962) 



approximately linear relation is known to hold between the 

 logarithm of the dose and the logarithm of the spleen index, the 

 log relative potency of the spleen cells of the test animal (i.e. 

 graft-bearing A mouse) to those of the standard (i.e. normal A 

 mouse) can be read as the horizontal distance between the fitted 

 parallel lines. 



It is evident that massive replacement of host lymphoid tissue 

 by donor tissue has occurred. The alternative view that the 

 reduction of activity of the host component was a long-lasting 

 sequela of irradiation was independently checked and excluded. 

 At the same time, three of the four analyses gave estimates of host 

 activity significantly in excess of zero, so that total replacement or 

 inactivation of host lymphoid cells had not occurred. 



