H-2 ANTIGENS OF THE MOUSE 6l 



desired product is present in small amount, when its chemical 

 nature is not known and when tests for activity leave much to be 

 desired. This situation holds for T, E and H antigens except that 

 we cannot be sure that they are present in small amount : indeed 

 there are indications to the contrary. The components of serum 

 and hence of ascitic fluid have been extensively studied and 

 although ascitic fluid is more complex in that it contains tumour 

 cell products, isolation from this source seemed to offer more 

 favourable prospects. With a knowledge of the chemical nature 

 and properties of an H-2 antigen, isolation from cells should 

 become a much less difficult task. Incubation of washed tumour 

 cell suspensions at 3 7° for 24 hours under toluene gives an extract 

 from which active H-2 inhibitors can be concentrated by the 

 dialysis steps described, but characterization of the ascitic fluid 

 product is still in progress and is to be completed before examina- 

 tion of the product of cell extraction. 



Information on homogeneity of the essentially insoluble 

 product is singularly difficult to obtain, but no evidence for the 

 presence of impurities has yet been found. It is not suggested, 

 however, that the least chemical entity able to carry H-2 specificity 

 has been obtained and it may yet be possible to remove parts of 

 the complex without impairing biological properties. Unfor- 

 tunately it has not been possible to obtain a true solution with full 

 activity, which would allow one to fractionate by the high- 

 resolution methods now available. 



The product, as it stands, is lipoprotein. The presence of a few 

 per cent of sugars leaves the possibility that activity is associated 

 with an independent carbohydrate present in small amount. This 

 seems very unlikely, however, in view of the inhibition activity 

 at high dilution (in spite of poor dispersity) and the fact that 

 most of the carbohydrate is present in the lipid fraction as 

 glycolipid. 



The protein component has been identified as an antigen in its 

 own right and probably dissociates from the H-2 antigen. It has a 



