284 F. M. BURNET 



sets of markers; the antigenic difference between WS and MEL and the 

 difference between presence and absence of neuropathogenicity, as tested by 

 intracerebral inoculation in 4-week mice. Baron and Jensen (1955) used in 

 addition thermostability of hemagglutinin and pathogenicity for mice by the 

 intranasal route. Burnet (1951) added pathogenicity for the chick embryo by 

 the chorioallantoic route and the reaction of heated virus hemagglutinin with 

 mucoprotein inhibitors. 



In subsequent discussion of the phenomena of linkage of characters, most 

 use will be made of the system based on the two influenza A viruses MEL and 

 WSE and to some extent applicable to all influenza A strains that we have 

 studied. These strains, as a result of a long series of passages since their 

 isolation in 1935 and 1933, respectively, differed in at least six characters. It 

 was convenient to represent the characters of MEL as ABCDEF and the 

 corresponding alternative characters of WSE as abcdef. As these symbols 

 have been widely used in our papers, it is desirable to provide some definition 

 of each of the characters. 



Aa. Serologically, as tested by hemagglutinin inhibition, the strains MEL 

 and WSE have only a small common component and they can be easily 

 differentiated with untreated antisera. By the use of appropriately absorbed 

 sera the difference can be made absolute. 



Bb. When MEL is heated at 56°C, there is little reduction in hemagglutinin 

 titer. WSE titers are progressively reduced, usually to one-half or less after 

 30 minutes. 



Cc. When certain strains of influenza virus are heated as crude allantoic 

 fluid at 55-56°C. for 30 minutes, they become highly susceptible to inhibition 

 of hemagglutination by a wide variety of mucoproteins, of which ovomucin 

 and semipurified human meconium are convenient examples. This conversion 

 to the "indicator state" character c is shown by WSE. MEL under the same 

 conditions remains quite insusceptible to inhibition, character C. 



Dd. This pair of characters similarly concerns the indicator state, but in 

 regard to a single inhibitory mucoprotein obtained from sheep salivary glands. 

 MEL cannot by any manipulation be made an indicator (character D) while 

 WSE is converted by simple heating (d). 



Ee. When WSE is inoculated on the chorioallantois of 12-day chick embryos, 

 easily visible pocks are produced and the virus invades the embryo which is 

 killed with gross hemorrhagic lesions in brain and other organs (character e). 

 MEL produces no more than a light granularity on the membrane, rarely 

 kills the embryo and produces no hemorrhagic lesions (E). 



Ff. Strain WSE is highly pathogenic by intranasal inoculation in lightly 

 anesthetized mice killing to a level of 1/10 AD (hemagglutinating dose). MEL 

 given in standard dose of one hemagglutinating unit produces patches of 

 nonlethal consolidation. 



