SEROLOGY OF VIRUSES 75 
ear. For this the ear is prepared by rubbing a very small amount of 
xylene or benzene on to its surface with a small wad of cotton wool. 
This is slightly irritant, causing the veins to fill with blood, and allows 
the bleeding to be done quickly and with a minimum of discomfort 
to the rabbit. A small cut is then made in the marginal vein near the 
base of the ear, using a small, very sharp instrument or a sharp piece 
of glass tubing. After sufficient blood has been taken, the flow can 
be stopped by applying slight pressure, and the cut sealed by a small 
amount of collodion dissolved in alcohol-ether. 
The cuts for later bleedings are made successively nearer the tip of 
the ear. The blood is collected in a tube and left for some hours to 
clot. The serum is poured off and centrifuged to remove any remaining 
blood cells. 
About 40 to 50 ml of blood can be taken on three successive days, 
followed by a further single bleeding at the end of a week. This 
gives about 100 ml of serum from one injection. The same rabbit 
after a rest period of a few weeks can then be used again. 
Antiserum prepared in this way is not sterile and will deteriorate 
unless stored under conditions which prevent bacterial growth. This 
may be prevented either by keeping the material frozen or by the 
addition of a few drops of chloroform. 
In either case it is advisable to keep the serum as cold as possible 
(Markham, Matthews, and Smith, 1948). 
The fact that a plant virus will react with its own antiserum and 
with no other, by forming a precipitate when it comes in contact 
with it, is of great use in identifying a virus quickly without the 
necessity of making inoculation tests to “indicator” plants, and then 
having to await the results. The precipitin test is now much used in a 
practical manner to identify viruses, especially in the testing of potato 
plants, where it is important that the plants should be free of virus 
(Chester, 1937). The following considerations apply particularly 
to the large-scale testing of potato plants, grown for seed, for the 
presence of virus X. 
One rabbit from one series of bleedings can give about 100 ml of 
antiserum. If o-s-ml quantities of a dilution of 1 in 50 of the antiserum 
are used, the amount of antiserum is sufficient for 10,000 single plant 
tests. If groups of 10 plants were tested, 100 ml of antiserum would 
be sufficient to test 100,000 plants. 
There are certain strains of potato virus X which give practically 
no symptoms on the usual test plants and could easily be missed in 
