Inoculation of the embryo-* witli virus for vaccine jiroduclion. 



lasted through the laying year. Individual handling- of the birds was not 

 necessary. 



Blood samples were taken from the chicks at the time of vaccination. 

 An attempt was made to procure samples at four-week intervals thereafter. 

 As the volume increased, however, samples were taken from different flocks 

 at different ages by random sampling. 



Explanation of Serological Tests 



The H. I. test emploN'ed in this work has been tised as a method of 

 determining antigenic response to vaccination with the Vt^ strain of New- 

 castle virus. This test is of the beta type, described l)y F. S. Markham. It 

 has been shown by challenge with a known virulent strain of virus that 

 negative tests are not a reliable indication of susceptibility. 



The geometric mean titre is a metlTod used in experimental and field 

 studies in serology, and is employed wlien it is desired to compare one 

 group with another. Unlike the average, which may be entirely misleading, 

 the geometric mean titre has a hxed value. 



The serum n.eutrali/.ation test used was a variation of the test described 

 Ijy f'^abricant (4). A serum dilution of 1 :10 was made and mixed with 10, 

 100. and 1000 neutralizing doses of eml^rvo-adapted infectious bronchitis 

 virus. Three embryos per diliuion were used. Results were considered pos- 

 itive when two out of three emI)ryos in the dilution of 100 or more neutra- 

 lizing doses lived. 



The same test was used in the detection of Newcastle antibodies, 

 except, of course, an embryo-adapted strain of Newcastle virus was used 

 as antigen. 



In challenging for infectious l)ronchitis, a field strain of bronchitis 

 \irus was used. This strain underwent a bird titre. It was found that a di- 

 lution of 10^*^ was the end point of infectivity in susceptible birds. A di- 

 lution of 10~-\, or 1000 infective doses, was applied. .2 cc. intranasally and 

 .2 cc. intratracheallv. 



